| Message ID | 20240226120321.1055731-3-ryan.roberts@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Address some contpte nits | expand |
On 26.02.24 13:03, Ryan Roberts wrote: > Make clear the atmicity/consistency requirements of the API and how we > achieve them. > > Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> > --- > arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- > 1 file changed, 14 insertions(+), 10 deletions(-) > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index be0a226c4ff9..1b64b4c3f8bf 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); > pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) > { > /* > - * Gather access/dirty bits, which may be populated in any of the ptes > - * of the contig range. We may not be holding the PTL, so any contiguous > - * range may be unfolded/modified/refolded under our feet. Therefore we > - * ensure we read a _consistent_ contpte range by checking that all ptes > - * in the range are valid and have CONT_PTE set, that all pfns are > - * contiguous and that all pgprots are the same (ignoring access/dirty). > - * If we find a pte that is not consistent, then we must be racing with > - * an update so start again. If the target pte does not have CONT_PTE > - * set then that is considered consistent on its own because it is not > - * part of a contpte range. > + * The ptep_get_lockless() API requires us to read and return *orig_ptep > + * so that it is self-consistent, without the PTL held, so we may be > + * racing with other threads modifying the pte. Usually a READ_ONCE() > + * would suffice, but for the contpte case, we also need to gather the > + * access and dirty bits from across all ptes in the contiguous block, > + * and we can't read all of those neighbouring ptes atomically, so any > + * contiguous range may be unfolded/modified/refolded under our feet. > + * Therefore we ensure we read a _consistent_ contpte range by checking > + * that all ptes in the range are valid and have CONT_PTE set, that all > + * pfns are contiguous and that all pgprots are the same (ignoring > + * access/dirty). If we find a pte that is not consistent, then we must > + * be racing with an update so start again. If the target pte does not > + * have CONT_PTE set then that is considered consistent on its own > + * because it is not part of a contpte range. > */ > > pgprot_t orig_prot; Reviewed-by: David Hildenbrand <david@redhat.com> In an ideal world, we'd really not rely on any accessed/dirty on the lockless path and remove contpte_ptep_get_lockless() completely :)
On 26/02/2024 12:30, David Hildenbrand wrote: > On 26.02.24 13:03, Ryan Roberts wrote: >> Make clear the atmicity/consistency requirements of the API and how we >> achieve them. >> >> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >> --- >> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >> 1 file changed, 14 insertions(+), 10 deletions(-) >> >> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >> index be0a226c4ff9..1b64b4c3f8bf 100644 >> --- a/arch/arm64/mm/contpte.c >> +++ b/arch/arm64/mm/contpte.c >> @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); >> pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >> { >> /* >> - * Gather access/dirty bits, which may be populated in any of the ptes >> - * of the contig range. We may not be holding the PTL, so any contiguous >> - * range may be unfolded/modified/refolded under our feet. Therefore we >> - * ensure we read a _consistent_ contpte range by checking that all ptes >> - * in the range are valid and have CONT_PTE set, that all pfns are >> - * contiguous and that all pgprots are the same (ignoring access/dirty). >> - * If we find a pte that is not consistent, then we must be racing with >> - * an update so start again. If the target pte does not have CONT_PTE >> - * set then that is considered consistent on its own because it is not >> - * part of a contpte range. >> + * The ptep_get_lockless() API requires us to read and return *orig_ptep >> + * so that it is self-consistent, without the PTL held, so we may be >> + * racing with other threads modifying the pte. Usually a READ_ONCE() >> + * would suffice, but for the contpte case, we also need to gather the >> + * access and dirty bits from across all ptes in the contiguous block, >> + * and we can't read all of those neighbouring ptes atomically, so any >> + * contiguous range may be unfolded/modified/refolded under our feet. >> + * Therefore we ensure we read a _consistent_ contpte range by checking >> + * that all ptes in the range are valid and have CONT_PTE set, that all >> + * pfns are contiguous and that all pgprots are the same (ignoring >> + * access/dirty). If we find a pte that is not consistent, then we must >> + * be racing with an update so start again. If the target pte does not >> + * have CONT_PTE set then that is considered consistent on its own >> + * because it is not part of a contpte range. >> */ >> pgprot_t orig_prot; > > Reviewed-by: David Hildenbrand <david@redhat.com> Thanks! > > In an ideal world, we'd really not rely on any accessed/dirty on the lockless > path and remove contpte_ptep_get_lockless() completely :) Not sure if you saw my RFC to do exactly that? (well, it doesn't actually remove [contpte_]ptep_get_lockless() but it does remove all the callers). If you have any feedback, we could get this moving... https://lore.kernel.org/linux-mm/20240215121756.2734131-1-ryan.roberts@arm.com/ Thanks, Ryan
On 26.02.24 13:37, Ryan Roberts wrote: > On 26/02/2024 12:30, David Hildenbrand wrote: >> On 26.02.24 13:03, Ryan Roberts wrote: >>> Make clear the atmicity/consistency requirements of the API and how we >>> achieve them. >>> >>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>> --- >>> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >>> 1 file changed, 14 insertions(+), 10 deletions(-) >>> >>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>> index be0a226c4ff9..1b64b4c3f8bf 100644 >>> --- a/arch/arm64/mm/contpte.c >>> +++ b/arch/arm64/mm/contpte.c >>> @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); >>> pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >>> { >>> /* >>> - * Gather access/dirty bits, which may be populated in any of the ptes >>> - * of the contig range. We may not be holding the PTL, so any contiguous >>> - * range may be unfolded/modified/refolded under our feet. Therefore we >>> - * ensure we read a _consistent_ contpte range by checking that all ptes >>> - * in the range are valid and have CONT_PTE set, that all pfns are >>> - * contiguous and that all pgprots are the same (ignoring access/dirty). >>> - * If we find a pte that is not consistent, then we must be racing with >>> - * an update so start again. If the target pte does not have CONT_PTE >>> - * set then that is considered consistent on its own because it is not >>> - * part of a contpte range. >>> + * The ptep_get_lockless() API requires us to read and return *orig_ptep >>> + * so that it is self-consistent, without the PTL held, so we may be >>> + * racing with other threads modifying the pte. Usually a READ_ONCE() >>> + * would suffice, but for the contpte case, we also need to gather the >>> + * access and dirty bits from across all ptes in the contiguous block, >>> + * and we can't read all of those neighbouring ptes atomically, so any >>> + * contiguous range may be unfolded/modified/refolded under our feet. >>> + * Therefore we ensure we read a _consistent_ contpte range by checking >>> + * that all ptes in the range are valid and have CONT_PTE set, that all >>> + * pfns are contiguous and that all pgprots are the same (ignoring >>> + * access/dirty). If we find a pte that is not consistent, then we must >>> + * be racing with an update so start again. If the target pte does not >>> + * have CONT_PTE set then that is considered consistent on its own >>> + * because it is not part of a contpte range. >>> */ >>> pgprot_t orig_prot; >> >> Reviewed-by: David Hildenbrand <david@redhat.com> > > Thanks! > >> >> In an ideal world, we'd really not rely on any accessed/dirty on the lockless >> path and remove contpte_ptep_get_lockless() completely :) > > Not sure if you saw my RFC to do exactly that? (well, it doesn't actually remove > [contpte_]ptep_get_lockless() but it does remove all the callers). If you have > any feedback, we could get this moving... Yes, I saw it. Hoping we can get that in and then maybe remove the contpte_get_lockless() once all callers are gone :) ... on my todo list.
On 2/26/24 04:03, Ryan Roberts wrote: Hi Ryan! > Make clear the atmicity/consistency requirements of the API and how we "atomicity" > achieve them. > > Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> > --- > arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- > 1 file changed, 14 insertions(+), 10 deletions(-) > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index be0a226c4ff9..1b64b4c3f8bf 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); > pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) > { > /* > - * Gather access/dirty bits, which may be populated in any of the ptes > - * of the contig range. We may not be holding the PTL, so any contiguous > - * range may be unfolded/modified/refolded under our feet. Therefore we > - * ensure we read a _consistent_ contpte range by checking that all ptes > - * in the range are valid and have CONT_PTE set, that all pfns are > - * contiguous and that all pgprots are the same (ignoring access/dirty). > - * If we find a pte that is not consistent, then we must be racing with > - * an update so start again. If the target pte does not have CONT_PTE > - * set then that is considered consistent on its own because it is not > - * part of a contpte range. > + * The ptep_get_lockless() API requires us to read and return *orig_ptep > + * so that it is self-consistent, without the PTL held, so we may be > + * racing with other threads modifying the pte. Usually a READ_ONCE() > + * would suffice, but for the contpte case, we also need to gather the > + * access and dirty bits from across all ptes in the contiguous block, > + * and we can't read all of those neighbouring ptes atomically, so any This still leaves a key detail unexplained: how the accessed and dirty bits are handled. The above raises the *problem*, but then talks about getting a consistent set of reads. But during those consistent reads, the HW could have dirtied or read a page. And this code here is only returning a single pte. So I'm still feeling vague about what we're trying to say about accessed and dirty bits. > + * contiguous range may be unfolded/modified/refolded under our feet. > + * Therefore we ensure we read a _consistent_ contpte range by checking > + * that all ptes in the range are valid and have CONT_PTE set, that all > + * pfns are contiguous and that all pgprots are the same (ignoring > + * access/dirty). If we find a pte that is not consistent, then we must > + * be racing with an update so start again. If the target pte does not > + * have CONT_PTE set then that is considered consistent on its own > + * because it is not part of a contpte range. > */ > > pgprot_t orig_prot; thanks,
On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: > Make clear the atmicity/consistency requirements of the API and how we > achieve them. > > Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> > --- > arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- > 1 file changed, 14 insertions(+), 10 deletions(-) > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index be0a226c4ff9..1b64b4c3f8bf 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); > pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) > { > /* > - * Gather access/dirty bits, which may be populated in any of the ptes > - * of the contig range. We may not be holding the PTL, so any contiguous > - * range may be unfolded/modified/refolded under our feet. Therefore we > - * ensure we read a _consistent_ contpte range by checking that all ptes > - * in the range are valid and have CONT_PTE set, that all pfns are > - * contiguous and that all pgprots are the same (ignoring access/dirty). > - * If we find a pte that is not consistent, then we must be racing with > - * an update so start again. If the target pte does not have CONT_PTE > - * set then that is considered consistent on its own because it is not > - * part of a contpte range. > + * The ptep_get_lockless() API requires us to read and return *orig_ptep > + * so that it is self-consistent, without the PTL held, so we may be > + * racing with other threads modifying the pte. Usually a READ_ONCE() > + * would suffice, but for the contpte case, we also need to gather the > + * access and dirty bits from across all ptes in the contiguous block, > + * and we can't read all of those neighbouring ptes atomically, so any > + * contiguous range may be unfolded/modified/refolded under our feet. > + * Therefore we ensure we read a _consistent_ contpte range by checking > + * that all ptes in the range are valid and have CONT_PTE set, that all > + * pfns are contiguous and that all pgprots are the same (ignoring > + * access/dirty). If we find a pte that is not consistent, then we must > + * be racing with an update so start again. If the target pte does not > + * have CONT_PTE set then that is considered consistent on its own > + * because it is not part of a contpte range. > */ I haven't had the time to properly think about this function but, depending on what its semantics are, we might not guarantee that, at the time of reading a pte, we have the correct dirty state from the other ptes in the range. Theoretical: let's say we read the first pte in the contig range and it's clean but further down there's a dirty one. Another (v)CPU breaks the contig range, sets the dirty bit everywhere, there's some pte_mkclean for all of them and they are collapsed into a contig range again. The function above on the first (v)CPU returns a clean pte when it should have actually been dirty at the time of read. Throughout the callers of this function, I couldn't find one where it matters. So I concluded that they don't need the dirty state. Normally the dirty state is passed to the page flags, so not lost after the pte has been cleaned.
On 01/03/2024 18:47, Catalin Marinas wrote: > On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: >> Make clear the atmicity/consistency requirements of the API and how we >> achieve them. >> >> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >> --- >> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >> 1 file changed, 14 insertions(+), 10 deletions(-) >> >> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >> index be0a226c4ff9..1b64b4c3f8bf 100644 >> --- a/arch/arm64/mm/contpte.c >> +++ b/arch/arm64/mm/contpte.c >> @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); >> pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >> { >> /* >> - * Gather access/dirty bits, which may be populated in any of the ptes >> - * of the contig range. We may not be holding the PTL, so any contiguous >> - * range may be unfolded/modified/refolded under our feet. Therefore we >> - * ensure we read a _consistent_ contpte range by checking that all ptes >> - * in the range are valid and have CONT_PTE set, that all pfns are >> - * contiguous and that all pgprots are the same (ignoring access/dirty). >> - * If we find a pte that is not consistent, then we must be racing with >> - * an update so start again. If the target pte does not have CONT_PTE >> - * set then that is considered consistent on its own because it is not >> - * part of a contpte range. >> + * The ptep_get_lockless() API requires us to read and return *orig_ptep >> + * so that it is self-consistent, without the PTL held, so we may be >> + * racing with other threads modifying the pte. Usually a READ_ONCE() >> + * would suffice, but for the contpte case, we also need to gather the >> + * access and dirty bits from across all ptes in the contiguous block, >> + * and we can't read all of those neighbouring ptes atomically, so any >> + * contiguous range may be unfolded/modified/refolded under our feet. >> + * Therefore we ensure we read a _consistent_ contpte range by checking >> + * that all ptes in the range are valid and have CONT_PTE set, that all >> + * pfns are contiguous and that all pgprots are the same (ignoring >> + * access/dirty). If we find a pte that is not consistent, then we must >> + * be racing with an update so start again. If the target pte does not >> + * have CONT_PTE set then that is considered consistent on its own >> + * because it is not part of a contpte range. >> */ > > I haven't had the time to properly think about this function but, > depending on what its semantics are, we might not guarantee that, at the > time of reading a pte, we have the correct dirty state from the other > ptes in the range. > > Theoretical: let's say we read the first pte in the contig range and > it's clean but further down there's a dirty one. Another (v)CPU breaks > the contig range, sets the dirty bit everywhere, there's some > pte_mkclean for all of them and they are collapsed into a contig range > again. The function above on the first (v)CPU returns a clean pte when > it should have actually been dirty at the time of read. But I think that still conforms to semantics of the function. If you had the same situation with a non-contpte mapping, the first thread may read the PTE at any time; when it's dirty, or after its been cleaned by the other thread. It's inherrently racy. All that matters is that what is returned is _consistent_; you don't want to be in a position where the first read of the block is mapping one folio, then by the time all the access/dirty bits are read, the mapping is actually to a different folio - that's an example of being inconsistent. > > Throughout the callers of this function, I couldn't find one where it > matters. So I concluded that they don't need the dirty state. Normally > the dirty state is passed to the page flags, so not lost after the pte > has been cleaned. I agree we can simplify the semantics. But I think its better done in a separate series (which I previously linked). What's the bottom line here? Are you ok with this comment as a short term solution for now, or do you want something more radical (i.e. push to get the series that does these simplifications reviewed and in time for v6.9). I still believe the current ptep_get_lockless() implementation is correct. So given I have a plan to simplify in the long run, I hope we can still get this series into v6.9 as planned. Thanks, Ryan
On Mon, Mar 04, 2024 at 12:54:23PM +0000, Ryan Roberts wrote: > On 01/03/2024 18:47, Catalin Marinas wrote: > > On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: > >> Make clear the atmicity/consistency requirements of the API and how we > >> achieve them. > >> > >> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ > >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> > >> --- > >> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- > >> 1 file changed, 14 insertions(+), 10 deletions(-) [...] > > Throughout the callers of this function, I couldn't find one where it > > matters. So I concluded that they don't need the dirty state. Normally > > the dirty state is passed to the page flags, so not lost after the pte > > has been cleaned. > > I agree we can simplify the semantics. But I think its better done in a separate > series (which I previously linked). > > What's the bottom line here? Are you ok with this comment as a short term > solution for now, or do you want something more radical (i.e. push to get the > series that does these simplifications reviewed and in time for v6.9). > > I still believe the current ptep_get_lockless() implementation is correct. So > given I have a plan to simplify in the long run, I hope we can still get this > series into v6.9 as planned. Yes, I'm fine with this patch. Assuming Andrew picked them up: Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> I'd like to get the simplification in as well at some point as I think our ptep_get_lockless() is unnecessarily complex for most use-cases.
On 04/03/2024 17:37, Catalin Marinas wrote: > On Mon, Mar 04, 2024 at 12:54:23PM +0000, Ryan Roberts wrote: >> On 01/03/2024 18:47, Catalin Marinas wrote: >>> On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: >>>> Make clear the atmicity/consistency requirements of the API and how we >>>> achieve them. >>>> >>>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>>> --- >>>> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >>>> 1 file changed, 14 insertions(+), 10 deletions(-) > [...] >>> Throughout the callers of this function, I couldn't find one where it >>> matters. So I concluded that they don't need the dirty state. Normally >>> the dirty state is passed to the page flags, so not lost after the pte >>> has been cleaned. >> >> I agree we can simplify the semantics. But I think its better done in a separate >> series (which I previously linked). >> >> What's the bottom line here? Are you ok with this comment as a short term >> solution for now, or do you want something more radical (i.e. push to get the >> series that does these simplifications reviewed and in time for v6.9). >> >> I still believe the current ptep_get_lockless() implementation is correct. So >> given I have a plan to simplify in the long run, I hope we can still get this >> series into v6.9 as planned. > > Yes, I'm fine with this patch. Assuming Andrew picked them up: > > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Thanks! Yes, he did - they are in mm-unstable. > > I'd like to get the simplification in as well at some point as I think > our ptep_get_lockless() is unnecessarily complex for most use-cases. Yes, I'll keep pushing it. I know DavidH is keen for it.
On 04.03.24 19:40, Ryan Roberts wrote: > On 04/03/2024 17:37, Catalin Marinas wrote: >> On Mon, Mar 04, 2024 at 12:54:23PM +0000, Ryan Roberts wrote: >>> On 01/03/2024 18:47, Catalin Marinas wrote: >>>> On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: >>>>> Make clear the atmicity/consistency requirements of the API and how we >>>>> achieve them. >>>>> >>>>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>>>> --- >>>>> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >>>>> 1 file changed, 14 insertions(+), 10 deletions(-) >> [...] >>>> Throughout the callers of this function, I couldn't find one where it >>>> matters. So I concluded that they don't need the dirty state. Normally >>>> the dirty state is passed to the page flags, so not lost after the pte >>>> has been cleaned. >>> >>> I agree we can simplify the semantics. But I think its better done in a separate >>> series (which I previously linked). >>> >>> What's the bottom line here? Are you ok with this comment as a short term >>> solution for now, or do you want something more radical (i.e. push to get the >>> series that does these simplifications reviewed and in time for v6.9). >>> >>> I still believe the current ptep_get_lockless() implementation is correct. So >>> given I have a plan to simplify in the long run, I hope we can still get this >>> series into v6.9 as planned. >> >> Yes, I'm fine with this patch. Assuming Andrew picked them up: >> >> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> > > Thanks! Yes, he did - they are in mm-unstable. > >> >> I'd like to get the simplification in as well at some point as I think >> our ptep_get_lockless() is unnecessarily complex for most use-cases. > > Yes, I'll keep pushing it. I know DavidH is keen for it. Maybe just sent a v1 (after the merge window?) if there is no further feedback. I still want to look into the details, but it's stuck deep in my inbox I'm afraid :)
On 04/03/2024 22:04, David Hildenbrand wrote: > On 04.03.24 19:40, Ryan Roberts wrote: >> On 04/03/2024 17:37, Catalin Marinas wrote: >>> On Mon, Mar 04, 2024 at 12:54:23PM +0000, Ryan Roberts wrote: >>>> On 01/03/2024 18:47, Catalin Marinas wrote: >>>>> On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: >>>>>> Make clear the atmicity/consistency requirements of the API and how we >>>>>> achieve them. >>>>>> >>>>>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>>>>> --- >>>>>> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >>>>>> 1 file changed, 14 insertions(+), 10 deletions(-) >>> [...] >>>>> Throughout the callers of this function, I couldn't find one where it >>>>> matters. So I concluded that they don't need the dirty state. Normally >>>>> the dirty state is passed to the page flags, so not lost after the pte >>>>> has been cleaned. >>>> >>>> I agree we can simplify the semantics. But I think its better done in a >>>> separate >>>> series (which I previously linked). >>>> >>>> What's the bottom line here? Are you ok with this comment as a short term >>>> solution for now, or do you want something more radical (i.e. push to get the >>>> series that does these simplifications reviewed and in time for v6.9). >>>> >>>> I still believe the current ptep_get_lockless() implementation is correct. So >>>> given I have a plan to simplify in the long run, I hope we can still get this >>>> series into v6.9 as planned. >>> >>> Yes, I'm fine with this patch. Assuming Andrew picked them up: >>> >>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> >> >> Thanks! Yes, he did - they are in mm-unstable. >> >>> >>> I'd like to get the simplification in as well at some point as I think >>> our ptep_get_lockless() is unnecessarily complex for most use-cases. >> >> Yes, I'll keep pushing it. I know DavidH is keen for it. > > Maybe just sent a v1 (after the merge window?) if there is no further feedback. > I still want to look into the details, but it's stuck deep in my inbox I'm > afraid :) Yep will do! I will also add the final patch to actually remove ptep_get_lockless() since it is no longer used by the end of the series.
On 04/03/2024 22:04, David Hildenbrand wrote: > On 04.03.24 19:40, Ryan Roberts wrote: >> On 04/03/2024 17:37, Catalin Marinas wrote: >>> On Mon, Mar 04, 2024 at 12:54:23PM +0000, Ryan Roberts wrote: >>>> On 01/03/2024 18:47, Catalin Marinas wrote: >>>>> On Mon, Feb 26, 2024 at 12:03:21PM +0000, Ryan Roberts wrote: >>>>>> Make clear the atmicity/consistency requirements of the API and how we >>>>>> achieve them. >>>>>> >>>>>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>>>>> --- >>>>>> arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >>>>>> 1 file changed, 14 insertions(+), 10 deletions(-) >>> [...] >>>>> Throughout the callers of this function, I couldn't find one where it >>>>> matters. So I concluded that they don't need the dirty state. Normally >>>>> the dirty state is passed to the page flags, so not lost after the pte >>>>> has been cleaned. >>>> >>>> I agree we can simplify the semantics. But I think its better done in a >>>> separate >>>> series (which I previously linked). >>>> >>>> What's the bottom line here? Are you ok with this comment as a short term >>>> solution for now, or do you want something more radical (i.e. push to get the >>>> series that does these simplifications reviewed and in time for v6.9). >>>> >>>> I still believe the current ptep_get_lockless() implementation is correct. So >>>> given I have a plan to simplify in the long run, I hope we can still get this >>>> series into v6.9 as planned. >>> >>> Yes, I'm fine with this patch. Assuming Andrew picked them up: >>> >>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> >> >> Thanks! Yes, he did - they are in mm-unstable. >> >>> >>> I'd like to get the simplification in as well at some point as I think >>> our ptep_get_lockless() is unnecessarily complex for most use-cases. >> >> Yes, I'll keep pushing it. I know DavidH is keen for it. > > Maybe just sent a v1 (after the merge window?) if there is no further feedback. > I still want to look into the details, but it's stuck deep in my inbox I'm > afraid :) Yep will do! I will also add the final patch to actually remove ptep_get_lockless() since it is no longer used by the end of the series.
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index be0a226c4ff9..1b64b4c3f8bf 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) { /* - * Gather access/dirty bits, which may be populated in any of the ptes - * of the contig range. We may not be holding the PTL, so any contiguous - * range may be unfolded/modified/refolded under our feet. Therefore we - * ensure we read a _consistent_ contpte range by checking that all ptes - * in the range are valid and have CONT_PTE set, that all pfns are - * contiguous and that all pgprots are the same (ignoring access/dirty). - * If we find a pte that is not consistent, then we must be racing with - * an update so start again. If the target pte does not have CONT_PTE - * set then that is considered consistent on its own because it is not - * part of a contpte range. + * The ptep_get_lockless() API requires us to read and return *orig_ptep + * so that it is self-consistent, without the PTL held, so we may be + * racing with other threads modifying the pte. Usually a READ_ONCE() + * would suffice, but for the contpte case, we also need to gather the + * access and dirty bits from across all ptes in the contiguous block, + * and we can't read all of those neighbouring ptes atomically, so any + * contiguous range may be unfolded/modified/refolded under our feet. + * Therefore we ensure we read a _consistent_ contpte range by checking + * that all ptes in the range are valid and have CONT_PTE set, that all + * pfns are contiguous and that all pgprots are the same (ignoring + * access/dirty). If we find a pte that is not consistent, then we must + * be racing with an update so start again. If the target pte does not + * have CONT_PTE set then that is considered consistent on its own + * because it is not part of a contpte range. */ pgprot_t orig_prot;
Make clear the atmicity/consistency requirements of the API and how we achieve them. Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> --- arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- 1 file changed, 14 insertions(+), 10 deletions(-)