| Message ID | 20210607055131.156184-2-aneesh.kumar@linux.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Speedup mremap on ppc64 | expand |
On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote: > CPU 1 CPU 2 CPU 3 > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > mmap_write_lock_killable() > > addr = old_addr > lock(pte_ptl) > lock(pmd_ptl) > pmd = *old_pmd > pmd_clear(old_pmd) > flush_tlb_range(old_addr) > > *new_pmd = pmd > *new_addr = 10; and fills > TLB with new addr > and old pfn > > unlock(pmd_ptl) > ptep_clear_flush() > old pfn is free. > Stale TLB entry > > Fix this race by holding pmd lock in pageout. This still doesn't handle the race > between MOVE_PUD and pageout. > > Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") > Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> This seems very wrong to me, to require another level of locking in the rmap lookup, just to fix some new pagetable games in mremap. But Linus asked "Am I missing something?": neither of you have mentioned mremap's take_rmap_locks(), so I hope that already meets your need. And if it needs to be called more often than before (see "need_rmap_locks"), that's probably okay. Hugh > --- > include/linux/rmap.h | 9 ++++++--- > mm/page_vma_mapped.c | 36 ++++++++++++++++++------------------ > 2 files changed, 24 insertions(+), 21 deletions(-) > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h > index def5c62c93b3..272ab0c2b60b 100644 > --- a/include/linux/rmap.h > +++ b/include/linux/rmap.h > @@ -207,7 +207,8 @@ struct page_vma_mapped_walk { > unsigned long address; > pmd_t *pmd; > pte_t *pte; > - spinlock_t *ptl; > + spinlock_t *pte_ptl; > + spinlock_t *pmd_ptl; > unsigned int flags; > }; > > @@ -216,8 +217,10 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw) > /* HugeTLB pte is set to the relevant page table entry without pte_mapped. */ > if (pvmw->pte && !PageHuge(pvmw->page)) > pte_unmap(pvmw->pte); > - if (pvmw->ptl) > - spin_unlock(pvmw->ptl); > + if (pvmw->pte_ptl) > + spin_unlock(pvmw->pte_ptl); > + if (pvmw->pmd_ptl) > + spin_unlock(pvmw->pmd_ptl); > } > > bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw); > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index 2cf01d933f13..87a2c94c7e27 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -47,8 +47,10 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw) > return false; > } > } > - pvmw->ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd); > - spin_lock(pvmw->ptl); > + if (USE_SPLIT_PTE_PTLOCKS) { > + pvmw->pte_ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd); > + spin_lock(pvmw->pte_ptl); > + } > return true; > } > > @@ -162,8 +164,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > if (!pvmw->pte) > return false; > > - pvmw->ptl = huge_pte_lockptr(page_hstate(page), mm, pvmw->pte); > - spin_lock(pvmw->ptl); > + pvmw->pte_ptl = huge_pte_lockptr(page_hstate(page), mm, pvmw->pte); > + spin_lock(pvmw->pte_ptl); > if (!check_pte(pvmw)) > return not_found(pvmw); > return true; > @@ -179,6 +181,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > if (!pud_present(*pud)) > return false; > pvmw->pmd = pmd_offset(pud, pvmw->address); > + pvmw->pmd_ptl = pmd_lock(mm, pvmw->pmd); > /* > * Make sure the pmd value isn't cached in a register by the > * compiler and used as a stale value after we've observed a > @@ -186,7 +189,6 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > */ > pmde = READ_ONCE(*pvmw->pmd); > if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) { > - pvmw->ptl = pmd_lock(mm, pvmw->pmd); > if (likely(pmd_trans_huge(*pvmw->pmd))) { > if (pvmw->flags & PVMW_MIGRATION) > return not_found(pvmw); > @@ -206,14 +208,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > } > } > return not_found(pvmw); > - } else { > - /* THP pmd was split under us: handle on pte level */ > - spin_unlock(pvmw->ptl); > - pvmw->ptl = NULL; > } > - } else if (!pmd_present(pmde)) { > - return false; > - } > + } else if (!pmd_present(pmde)) > + return not_found(pvmw); > + > if (!map_pte(pvmw)) > goto next_pte; > while (1) { > @@ -233,19 +231,21 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > /* Did we cross page table boundary? */ > if (pvmw->address % PMD_SIZE == 0) { > pte_unmap(pvmw->pte); > - if (pvmw->ptl) { > - spin_unlock(pvmw->ptl); > - pvmw->ptl = NULL; > + if (pvmw->pte_ptl) { > + spin_unlock(pvmw->pte_ptl); > + pvmw->pte_ptl = NULL; > } > + spin_unlock(pvmw->pmd_ptl); > + pvmw->pmd_ptl = NULL; > goto restart; > } else { > pvmw->pte++; > } > } while (pte_none(*pvmw->pte)); > > - if (!pvmw->ptl) { > - pvmw->ptl = pte_lockptr(mm, pvmw->pmd); > - spin_lock(pvmw->ptl); > + if (USE_SPLIT_PTE_PTLOCKS && !pvmw->pte_ptl) { > + pvmw->pte_ptl = pte_lockptr(mm, pvmw->pmd); > + spin_lock(pvmw->pte_ptl); > } > } > } > -- > 2.31.1
Hi Hugh, Hugh Dickins <hughd@google.com> writes: > On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote: > >> CPU 1 CPU 2 CPU 3 >> >> mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one >> >> mmap_write_lock_killable() >> >> addr = old_addr >> lock(pte_ptl) >> lock(pmd_ptl) >> pmd = *old_pmd >> pmd_clear(old_pmd) >> flush_tlb_range(old_addr) >> >> *new_pmd = pmd >> *new_addr = 10; and fills >> TLB with new addr >> and old pfn >> >> unlock(pmd_ptl) >> ptep_clear_flush() >> old pfn is free. >> Stale TLB entry >> >> Fix this race by holding pmd lock in pageout. This still doesn't handle the race >> between MOVE_PUD and pageout. >> >> Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") >> Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> > > This seems very wrong to me, to require another level of locking in the > rmap lookup, just to fix some new pagetable games in mremap. > > But Linus asked "Am I missing something?": neither of you have mentioned > mremap's take_rmap_locks(), so I hope that already meets your need. And > if it needs to be called more often than before (see "need_rmap_locks"), > that's probably okay. > > Hugh > Thanks for reviewing the change. I missed the rmap lock in the code path. How about the below change? mm/mremap: hold the rmap lock in write mode when moving page table entries. To avoid a race between rmap walk and mremap, mremap does take_rmap_locks(). The lock was taken to ensure that rmap walk don't miss a page table entry due to PTE moves via move_pagetables(). The kernel does further optimization of this lock such that if we are going to find the newly added vma after the old vma, the rmap lock is not taken. This is because rmap walk would find the vmas in the same order and if we don't find the page table attached to older vma we would find it with the new vma which we would iterate later. The actual lifetime of the page is still controlled by the PTE lock. This patch updates the locking requirement to handle another race condition explained below with optimized mremap:: Optmized PMD move CPU 1 CPU 2 CPU 3 mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one mmap_write_lock_killable() addr = old_addr lock(pte_ptl) lock(pmd_ptl) pmd = *old_pmd pmd_clear(old_pmd) flush_tlb_range(old_addr) *new_pmd = pmd *new_addr = 10; and fills TLB with new addr and old pfn unlock(pmd_ptl) ptep_clear_flush() old pfn is free. Stale TLB entry Optmized PUD move: CPU 1 CPU 2 CPU 3 mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one mmap_write_lock_killable() addr = old_addr lock(pte_ptl) lock(pud_ptl) pud = *old_pud pud_clear(old_pud) flush_tlb_range(old_addr) *new_pud = pud *new_addr = 10; and fills TLB with new addr and old pfn unlock(pud_ptl) ptep_clear_flush() old pfn is free. Stale TLB entry Both the above race condition can be fixed if we force mremap path to take rmap lock. Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> diff --git a/mm/mremap.c b/mm/mremap.c index 9cd352fb9cf8..f12df630fb37 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -517,7 +517,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, } else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) { if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr, - old_pud, new_pud, need_rmap_locks)) + old_pud, new_pud, true)) continue; } @@ -544,7 +544,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, * moving at the PMD level if possible. */ if (move_pgt_entry(NORMAL_PMD, vma, old_addr, new_addr, - old_pmd, new_pmd, need_rmap_locks)) + old_pmd, new_pmd, true)) continue; }
On Tue, Jun 08, 2021 at 01:22:23PM +0530, Aneesh Kumar K.V wrote: > > Hi Hugh, > > Hugh Dickins <hughd@google.com> writes: > > > On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote: > > > >> CPU 1 CPU 2 CPU 3 > >> > >> mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > >> > >> mmap_write_lock_killable() > >> > >> addr = old_addr > >> lock(pte_ptl) > >> lock(pmd_ptl) > >> pmd = *old_pmd > >> pmd_clear(old_pmd) > >> flush_tlb_range(old_addr) > >> > >> *new_pmd = pmd > >> *new_addr = 10; and fills > >> TLB with new addr > >> and old pfn > >> > >> unlock(pmd_ptl) > >> ptep_clear_flush() > >> old pfn is free. > >> Stale TLB entry > >> > >> Fix this race by holding pmd lock in pageout. This still doesn't handle the race > >> between MOVE_PUD and pageout. > >> > >> Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") > >> Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com > >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> > > > > This seems very wrong to me, to require another level of locking in the > > rmap lookup, just to fix some new pagetable games in mremap. > > > > But Linus asked "Am I missing something?": neither of you have mentioned > > mremap's take_rmap_locks(), so I hope that already meets your need. And > > if it needs to be called more often than before (see "need_rmap_locks"), > > that's probably okay. > > > > Hugh > > > > Thanks for reviewing the change. I missed the rmap lock in the code > path. How about the below change? > > mm/mremap: hold the rmap lock in write mode when moving page table entries. > > To avoid a race between rmap walk and mremap, mremap does take_rmap_locks(). > The lock was taken to ensure that rmap walk don't miss a page table entry due to > PTE moves via move_pagetables(). The kernel does further optimization of > this lock such that if we are going to find the newly added vma after the > old vma, the rmap lock is not taken. This is because rmap walk would find the > vmas in the same order and if we don't find the page table attached to > older vma we would find it with the new vma which we would iterate later. > The actual lifetime of the page is still controlled by the PTE lock. > > This patch updates the locking requirement to handle another race condition > explained below with optimized mremap:: > > Optmized PMD move > > CPU 1 CPU 2 CPU 3 > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > mmap_write_lock_killable() > > addr = old_addr > lock(pte_ptl) > lock(pmd_ptl) > pmd = *old_pmd > pmd_clear(old_pmd) > flush_tlb_range(old_addr) > > *new_pmd = pmd > *new_addr = 10; and fills > TLB with new addr > and old pfn > > unlock(pmd_ptl) > ptep_clear_flush() > old pfn is free. > Stale TLB entry > > Optmized PUD move: > > CPU 1 CPU 2 CPU 3 > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > mmap_write_lock_killable() > > addr = old_addr > lock(pte_ptl) > lock(pud_ptl) > pud = *old_pud > pud_clear(old_pud) > flush_tlb_range(old_addr) > > *new_pud = pud > *new_addr = 10; and fills > TLB with new addr > and old pfn > > unlock(pud_ptl) > ptep_clear_flush() > old pfn is free. > Stale TLB entry > > Both the above race condition can be fixed if we force mremap path to take rmap lock. > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> Looks like it should be enough to address the race. It would be nice to understand what is performance overhead of the additional locking. Is it still faster to move single PMD page table under these locks comparing to moving PTE page table entries without the locks?
On 6/8/21 3:12 PM, Kirill A. Shutemov wrote: > On Tue, Jun 08, 2021 at 01:22:23PM +0530, Aneesh Kumar K.V wrote: >> >> Hi Hugh, >> >> Hugh Dickins <hughd@google.com> writes: >> >>> On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote: >>> >>>> CPU 1 CPU 2 CPU 3 >>>> >>>> mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one >>>> >>>> mmap_write_lock_killable() >>>> >>>> addr = old_addr >>>> lock(pte_ptl) >>>> lock(pmd_ptl) >>>> pmd = *old_pmd >>>> pmd_clear(old_pmd) >>>> flush_tlb_range(old_addr) >>>> >>>> *new_pmd = pmd >>>> *new_addr = 10; and fills >>>> TLB with new addr >>>> and old pfn >>>> >>>> unlock(pmd_ptl) >>>> ptep_clear_flush() >>>> old pfn is free. >>>> Stale TLB entry >>>> >>>> Fix this race by holding pmd lock in pageout. This still doesn't handle the race >>>> between MOVE_PUD and pageout. >>>> >>>> Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") >>>> Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com >>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> >>> >>> This seems very wrong to me, to require another level of locking in the >>> rmap lookup, just to fix some new pagetable games in mremap. >>> >>> But Linus asked "Am I missing something?": neither of you have mentioned >>> mremap's take_rmap_locks(), so I hope that already meets your need. And >>> if it needs to be called more often than before (see "need_rmap_locks"), >>> that's probably okay. >>> >>> Hugh >>> >> >> Thanks for reviewing the change. I missed the rmap lock in the code >> path. How about the below change? >> >> mm/mremap: hold the rmap lock in write mode when moving page table entries. >> >> To avoid a race between rmap walk and mremap, mremap does take_rmap_locks(). >> The lock was taken to ensure that rmap walk don't miss a page table entry due to >> PTE moves via move_pagetables(). The kernel does further optimization of >> this lock such that if we are going to find the newly added vma after the >> old vma, the rmap lock is not taken. This is because rmap walk would find the >> vmas in the same order and if we don't find the page table attached to >> older vma we would find it with the new vma which we would iterate later. >> The actual lifetime of the page is still controlled by the PTE lock. >> >> This patch updates the locking requirement to handle another race condition >> explained below with optimized mremap:: >> >> Optmized PMD move >> >> CPU 1 CPU 2 CPU 3 >> >> mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one >> >> mmap_write_lock_killable() >> >> addr = old_addr >> lock(pte_ptl) >> lock(pmd_ptl) >> pmd = *old_pmd >> pmd_clear(old_pmd) >> flush_tlb_range(old_addr) >> >> *new_pmd = pmd >> *new_addr = 10; and fills >> TLB with new addr >> and old pfn >> >> unlock(pmd_ptl) >> ptep_clear_flush() >> old pfn is free. >> Stale TLB entry >> >> Optmized PUD move: >> >> CPU 1 CPU 2 CPU 3 >> >> mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one >> >> mmap_write_lock_killable() >> >> addr = old_addr >> lock(pte_ptl) >> lock(pud_ptl) >> pud = *old_pud >> pud_clear(old_pud) >> flush_tlb_range(old_addr) >> >> *new_pud = pud >> *new_addr = 10; and fills >> TLB with new addr >> and old pfn >> >> unlock(pud_ptl) >> ptep_clear_flush() >> old pfn is free. >> Stale TLB entry >> >> Both the above race condition can be fixed if we force mremap path to take rmap lock. >> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> > > Looks like it should be enough to address the race. > > It would be nice to understand what is performance overhead of the > additional locking. Is it still faster to move single PMD page table under > these locks comparing to moving PTE page table entries without the locks? > The improvements provided by optimized mremap as captured in patch 11 is large. mremap HAVE_MOVE_PMD/PUD optimization time comparison for 1GB region: 1GB mremap - Source PTE-aligned, Destination PTE-aligned mremap time: 2292772ns 1GB mremap - Source PMD-aligned, Destination PMD-aligned mremap time: 1158928ns 1GB mremap - Source PUD-aligned, Destination PUD-aligned mremap time: 63886ns With additional locking, I haven't observed much change in those numbers. But that could also be because there is no contention on these locks when this test is run? -aneesh
On Tue, Jun 08, 2021 at 04:47:19PM +0530, Aneesh Kumar K.V wrote: > On 6/8/21 3:12 PM, Kirill A. Shutemov wrote: > > On Tue, Jun 08, 2021 at 01:22:23PM +0530, Aneesh Kumar K.V wrote: > > > > > > Hi Hugh, > > > > > > Hugh Dickins <hughd@google.com> writes: > > > > > > > On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote: > > > > > > > > > CPU 1 CPU 2 CPU 3 > > > > > > > > > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > > > > > > > > > mmap_write_lock_killable() > > > > > > > > > > addr = old_addr > > > > > lock(pte_ptl) > > > > > lock(pmd_ptl) > > > > > pmd = *old_pmd > > > > > pmd_clear(old_pmd) > > > > > flush_tlb_range(old_addr) > > > > > > > > > > *new_pmd = pmd > > > > > *new_addr = 10; and fills > > > > > TLB with new addr > > > > > and old pfn > > > > > > > > > > unlock(pmd_ptl) > > > > > ptep_clear_flush() > > > > > old pfn is free. > > > > > Stale TLB entry > > > > > > > > > > Fix this race by holding pmd lock in pageout. This still doesn't handle the race > > > > > between MOVE_PUD and pageout. > > > > > > > > > > Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") > > > > > Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com > > > > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> > > > > > > > > This seems very wrong to me, to require another level of locking in the > > > > rmap lookup, just to fix some new pagetable games in mremap. > > > > > > > > But Linus asked "Am I missing something?": neither of you have mentioned > > > > mremap's take_rmap_locks(), so I hope that already meets your need. And > > > > if it needs to be called more often than before (see "need_rmap_locks"), > > > > that's probably okay. > > > > > > > > Hugh > > > > > > > > > > Thanks for reviewing the change. I missed the rmap lock in the code > > > path. How about the below change? > > > > > > mm/mremap: hold the rmap lock in write mode when moving page table entries. > > > To avoid a race between rmap walk and mremap, mremap does take_rmap_locks(). > > > The lock was taken to ensure that rmap walk don't miss a page table entry due to > > > PTE moves via move_pagetables(). The kernel does further optimization of > > > this lock such that if we are going to find the newly added vma after the > > > old vma, the rmap lock is not taken. This is because rmap walk would find the > > > vmas in the same order and if we don't find the page table attached to > > > older vma we would find it with the new vma which we would iterate later. > > > The actual lifetime of the page is still controlled by the PTE lock. > > > This patch updates the locking requirement to handle another race condition > > > explained below with optimized mremap:: > > > Optmized PMD move > > > CPU 1 CPU 2 CPU 3 > > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > > mmap_write_lock_killable() > > > addr = old_addr > > > lock(pte_ptl) > > > lock(pmd_ptl) > > > pmd = *old_pmd > > > pmd_clear(old_pmd) > > > flush_tlb_range(old_addr) > > > *new_pmd = pmd > > > *new_addr = 10; and fills > > > TLB with new addr > > > and old pfn > > > unlock(pmd_ptl) > > > ptep_clear_flush() > > > old pfn is free. > > > Stale TLB entry > > > Optmized PUD move: > > > CPU 1 CPU 2 CPU 3 > > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > > mmap_write_lock_killable() > > > addr = old_addr > > > lock(pte_ptl) > > > lock(pud_ptl) > > > pud = *old_pud > > > pud_clear(old_pud) > > > flush_tlb_range(old_addr) > > > *new_pud = pud > > > *new_addr = 10; and fills > > > TLB with new addr > > > and old pfn > > > unlock(pud_ptl) > > > ptep_clear_flush() > > > old pfn is free. > > > Stale TLB entry > > > Both the above race condition can be fixed if we force mremap path to take rmap lock. > > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> > > > > Looks like it should be enough to address the race. > > > > It would be nice to understand what is performance overhead of the > > additional locking. Is it still faster to move single PMD page table under > > these locks comparing to moving PTE page table entries without the locks? > > > > The improvements provided by optimized mremap as captured in patch 11 is > large. > > mremap HAVE_MOVE_PMD/PUD optimization time comparison for 1GB region: > 1GB mremap - Source PTE-aligned, Destination PTE-aligned > mremap time: 2292772ns > 1GB mremap - Source PMD-aligned, Destination PMD-aligned > mremap time: 1158928ns > 1GB mremap - Source PUD-aligned, Destination PUD-aligned > mremap time: 63886ns > > With additional locking, I haven't observed much change in those numbers. > But that could also be because there is no contention on these locks when > this test is run? Okay, it's good enough: contention should not be common and it's okay to pay a price for correctness. Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
On Tue, 8 Jun 2021, Aneesh Kumar K.V wrote: > > mm/mremap: hold the rmap lock in write mode when moving page table entries. > > To avoid a race between rmap walk and mremap, mremap does take_rmap_locks(). > The lock was taken to ensure that rmap walk don't miss a page table entry due to > PTE moves via move_pagetables(). The kernel does further optimization of > this lock such that if we are going to find the newly added vma after the > old vma, the rmap lock is not taken. This is because rmap walk would find the > vmas in the same order and if we don't find the page table attached to > older vma we would find it with the new vma which we would iterate later. > The actual lifetime of the page is still controlled by the PTE lock. > > This patch updates the locking requirement to handle another race condition > explained below with optimized mremap:: > > Optmized PMD move > > CPU 1 CPU 2 CPU 3 > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > mmap_write_lock_killable() > > addr = old_addr > lock(pte_ptl) > lock(pmd_ptl) > pmd = *old_pmd > pmd_clear(old_pmd) > flush_tlb_range(old_addr) > > *new_pmd = pmd > *new_addr = 10; and fills > TLB with new addr > and old pfn > > unlock(pmd_ptl) > ptep_clear_flush() > old pfn is free. > Stale TLB entry > The PUD example below is mainly a waste a space and time: "Optimized PUD move suffers from a similar race." would be better. > Optmized PUD move: > > CPU 1 CPU 2 CPU 3 > > mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one > > mmap_write_lock_killable() > > addr = old_addr > lock(pte_ptl) > lock(pud_ptl) > pud = *old_pud > pud_clear(old_pud) > flush_tlb_range(old_addr) > > *new_pud = pud > *new_addr = 10; and fills > TLB with new addr > and old pfn > > unlock(pud_ptl) > ptep_clear_flush() > old pfn is free. > Stale TLB entry > > Both the above race condition can be fixed if we force mremap path to take rmap lock. > Don't forget the Fixes and Link you had in the previous version: Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> Thanks, this is orders of magnitude better! Acked-by: Hugh Dickins <hughd@google.com> > > diff --git a/mm/mremap.c b/mm/mremap.c > index 9cd352fb9cf8..f12df630fb37 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -517,7 +517,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > } else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) { > > if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr, > - old_pud, new_pud, need_rmap_locks)) > + old_pud, new_pud, true)) > continue; > } > > @@ -544,7 +544,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > * moving at the PMD level if possible. > */ > if (move_pgt_entry(NORMAL_PMD, vma, old_addr, new_addr, > - old_pmd, new_pmd, need_rmap_locks)) > + old_pmd, new_pmd, true)) > continue; > } > >
diff --git a/include/linux/rmap.h b/include/linux/rmap.h index def5c62c93b3..272ab0c2b60b 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -207,7 +207,8 @@ struct page_vma_mapped_walk { unsigned long address; pmd_t *pmd; pte_t *pte; - spinlock_t *ptl; + spinlock_t *pte_ptl; + spinlock_t *pmd_ptl; unsigned int flags; }; @@ -216,8 +217,10 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw) /* HugeTLB pte is set to the relevant page table entry without pte_mapped. */ if (pvmw->pte && !PageHuge(pvmw->page)) pte_unmap(pvmw->pte); - if (pvmw->ptl) - spin_unlock(pvmw->ptl); + if (pvmw->pte_ptl) + spin_unlock(pvmw->pte_ptl); + if (pvmw->pmd_ptl) + spin_unlock(pvmw->pmd_ptl); } bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw); diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index 2cf01d933f13..87a2c94c7e27 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -47,8 +47,10 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw) return false; } } - pvmw->ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd); - spin_lock(pvmw->ptl); + if (USE_SPLIT_PTE_PTLOCKS) { + pvmw->pte_ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd); + spin_lock(pvmw->pte_ptl); + } return true; } @@ -162,8 +164,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) if (!pvmw->pte) return false; - pvmw->ptl = huge_pte_lockptr(page_hstate(page), mm, pvmw->pte); - spin_lock(pvmw->ptl); + pvmw->pte_ptl = huge_pte_lockptr(page_hstate(page), mm, pvmw->pte); + spin_lock(pvmw->pte_ptl); if (!check_pte(pvmw)) return not_found(pvmw); return true; @@ -179,6 +181,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) if (!pud_present(*pud)) return false; pvmw->pmd = pmd_offset(pud, pvmw->address); + pvmw->pmd_ptl = pmd_lock(mm, pvmw->pmd); /* * Make sure the pmd value isn't cached in a register by the * compiler and used as a stale value after we've observed a @@ -186,7 +189,6 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) */ pmde = READ_ONCE(*pvmw->pmd); if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) { - pvmw->ptl = pmd_lock(mm, pvmw->pmd); if (likely(pmd_trans_huge(*pvmw->pmd))) { if (pvmw->flags & PVMW_MIGRATION) return not_found(pvmw); @@ -206,14 +208,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) } } return not_found(pvmw); - } else { - /* THP pmd was split under us: handle on pte level */ - spin_unlock(pvmw->ptl); - pvmw->ptl = NULL; } - } else if (!pmd_present(pmde)) { - return false; - } + } else if (!pmd_present(pmde)) + return not_found(pvmw); + if (!map_pte(pvmw)) goto next_pte; while (1) { @@ -233,19 +231,21 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) /* Did we cross page table boundary? */ if (pvmw->address % PMD_SIZE == 0) { pte_unmap(pvmw->pte); - if (pvmw->ptl) { - spin_unlock(pvmw->ptl); - pvmw->ptl = NULL; + if (pvmw->pte_ptl) { + spin_unlock(pvmw->pte_ptl); + pvmw->pte_ptl = NULL; } + spin_unlock(pvmw->pmd_ptl); + pvmw->pmd_ptl = NULL; goto restart; } else { pvmw->pte++; } } while (pte_none(*pvmw->pte)); - if (!pvmw->ptl) { - pvmw->ptl = pte_lockptr(mm, pvmw->pmd); - spin_lock(pvmw->ptl); + if (USE_SPLIT_PTE_PTLOCKS && !pvmw->pte_ptl) { + pvmw->pte_ptl = pte_lockptr(mm, pvmw->pmd); + spin_lock(pvmw->pte_ptl); } } }
CPU 1 CPU 2 CPU 3 mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one mmap_write_lock_killable() addr = old_addr lock(pte_ptl) lock(pmd_ptl) pmd = *old_pmd pmd_clear(old_pmd) flush_tlb_range(old_addr) *new_pmd = pmd *new_addr = 10; and fills TLB with new addr and old pfn unlock(pmd_ptl) ptep_clear_flush() old pfn is free. Stale TLB entry Fix this race by holding pmd lock in pageout. This still doesn't handle the race between MOVE_PUD and pageout. Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> --- include/linux/rmap.h | 9 ++++++--- mm/page_vma_mapped.c | 36 ++++++++++++++++++------------------ 2 files changed, 24 insertions(+), 21 deletions(-)