| Message ID | 20200804175516.2511704-1-boris@bur.io (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] btrfs: change commit txn to end txn in subvol_setflags ioctl | expand |
On 2020/8/5 上午1:55, Boris Burkov wrote: > Currently, btrfs_ioctl_subvol_setflags forces a btrfs_commit_transaction > while holding subvol_sem. As a result, we have seen workloads where > calling `btrfs property set -ts <subvol> ro false` hangs waiting for a > legitimately slow commit. This gets even worse if the workload tries to > set flags on multiple subvolumes and the ioctls pile up on subvol_sem. > > Change the commit to a btrfs_end_transaction so that the ioctl can > return in a timely fashion and piggy back on a later commit. > > Signed-off-by: Boris Burkov <boris@bur.io> > --- > fs/btrfs/ioctl.c | 2 +- > fs/btrfs/transaction.c | 4 ++-- > 2 files changed, 3 insertions(+), 3 deletions(-) > > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c > index bd3511c5ca81..3ae484768ce7 100644 > --- a/fs/btrfs/ioctl.c > +++ b/fs/btrfs/ioctl.c > @@ -1985,7 +1985,7 @@ static noinline int btrfs_ioctl_subvol_setflags(struct file *file, > goto out_reset; > } > > - ret = btrfs_commit_transaction(trans); > + ret = btrfs_end_transaction(trans); This means the setflag is not committed to disk, and if a powerloss happens before a transaction commit, then the setflag operation just get lost. This means, previously if this ioctl returns, users can expect that the flag is always set no matter what, but now there is no guarantee. Personally I'm not sure if we really want that operation to be committed to disk. Maybe that transaction commit can be initialized in user space, so for multiple setflags, we only commit once, thus saves a lot of time. Thanks, Qu > > out_reset: > if (ret) > diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c > index 20c6ac1a5de7..1dc44209c2ae 100644 > --- a/fs/btrfs/transaction.c > +++ b/fs/btrfs/transaction.c > @@ -47,7 +47,7 @@ > * | Will wait for previous running transaction to completely finish if there > * | is one > * | > - * | Then one of the following happes: > + * | Then one of the following happens: > * | - Wait for all other trans handle holders to release. > * | The btrfs_commit_transaction() caller will do the commit work. > * | - Wait for current transaction to be committed by others. > @@ -60,7 +60,7 @@ > * | > * | To next stage: > * | Caller is chosen to commit transaction N, and all other trans handle > - * | haven been released. > + * | have been released. > * V > * Transaction N [[TRANS_STATE_COMMIT_DOING]] > * | >
On 8/4/20 6:48 PM, Qu Wenruo wrote: > > > On 2020/8/5 上午1:55, Boris Burkov wrote: >> Currently, btrfs_ioctl_subvol_setflags forces a btrfs_commit_transaction >> while holding subvol_sem. As a result, we have seen workloads where >> calling `btrfs property set -ts <subvol> ro false` hangs waiting for a >> legitimately slow commit. This gets even worse if the workload tries to >> set flags on multiple subvolumes and the ioctls pile up on subvol_sem. >> >> Change the commit to a btrfs_end_transaction so that the ioctl can >> return in a timely fashion and piggy back on a later commit. >> >> Signed-off-by: Boris Burkov <boris@bur.io> >> --- >> fs/btrfs/ioctl.c | 2 +- >> fs/btrfs/transaction.c | 4 ++-- >> 2 files changed, 3 insertions(+), 3 deletions(-) >> >> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c >> index bd3511c5ca81..3ae484768ce7 100644 >> --- a/fs/btrfs/ioctl.c >> +++ b/fs/btrfs/ioctl.c >> @@ -1985,7 +1985,7 @@ static noinline int btrfs_ioctl_subvol_setflags(struct file *file, >> goto out_reset; >> } >> >> - ret = btrfs_commit_transaction(trans); >> + ret = btrfs_end_transaction(trans); > > This means the setflag is not committed to disk, and if a powerloss > happens before a transaction commit, then the setflag operation just get > lost. > > This means, previously if this ioctl returns, users can expect that the > flag is always set no matter what, but now there is no guarantee. > > Personally I'm not sure if we really want that operation to be committed > to disk. > Maybe that transaction commit can be initialized in user space, so for > multiple setflags, we only commit once, thus saves a lot of time. > I'm of the opinion that we shouldn't be committing the transaction for stuff like this, unless there's a really good reason to. Especially given we're holding the subvol lock here, we should just do end_transaction. Thanks, Josef
On 05.08.2020 01:08 Josef Bacik wrote: > On 8/4/20 6:48 PM, Qu Wenruo wrote: >> >> >> On 2020/8/5 上午1:55, Boris Burkov wrote: >>> Currently, btrfs_ioctl_subvol_setflags forces a >>> btrfs_commit_transaction >>> while holding subvol_sem. As a result, we have seen workloads where >>> calling `btrfs property set -ts <subvol> ro false` hangs waiting for a >>> legitimately slow commit. This gets even worse if the workload tries to >>> set flags on multiple subvolumes and the ioctls pile up on subvol_sem. >>> >>> Change the commit to a btrfs_end_transaction so that the ioctl can >>> return in a timely fashion and piggy back on a later commit. >>> >>> Signed-off-by: Boris Burkov <boris@bur.io> >>> --- >>> fs/btrfs/ioctl.c | 2 +- >>> fs/btrfs/transaction.c | 4 ++-- >>> 2 files changed, 3 insertions(+), 3 deletions(-) >>> >>> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c >>> index bd3511c5ca81..3ae484768ce7 100644 >>> --- a/fs/btrfs/ioctl.c >>> +++ b/fs/btrfs/ioctl.c >>> @@ -1985,7 +1985,7 @@ static noinline int >>> btrfs_ioctl_subvol_setflags(struct file *file, >>> goto out_reset; >>> } >>> - ret = btrfs_commit_transaction(trans); >>> + ret = btrfs_end_transaction(trans); >> >> This means the setflag is not committed to disk, and if a powerloss >> happens before a transaction commit, then the setflag operation just get >> lost. >> >> This means, previously if this ioctl returns, users can expect that the >> flag is always set no matter what, but now there is no guarantee. >> >> Personally I'm not sure if we really want that operation to be committed >> to disk. >> Maybe that transaction commit can be initialized in user space, so for >> multiple setflags, we only commit once, thus saves a lot of time. >> > > I'm of the opinion that we shouldn't be committing the transaction for > stuff like this, unless there's a really good reason to. Especially > given we're holding the subvol lock here, we should just do > end_transaction. Thanks, From a user perspective I'd appreciate having the option to set it in a non-durable way (I have seen btrfs property sets hanging for a long time as well). But currently my application kind of depends on it being durable. Making it non-durable wouldn't break much and I guess the old behaviour could be emulated by a "btrfs fi sync <subvol>" afterwards, but idk how much other stuff depends on it being durable. Making it consistent with btrfs subvol del with the "-c" switch would be nice and consistent as well (and the -c switch could be done via IOC_SYNC after setting the properties).
On Wed, Aug 05, 2020 at 01:40:16PM +0000, Martin Raiber wrote: > On 05.08.2020 01:08 Josef Bacik wrote: > >On 8/4/20 6:48 PM, Qu Wenruo wrote: > >> > >> > >>On 2020/8/5 上午1:55, Boris Burkov wrote: > >>>Currently, btrfs_ioctl_subvol_setflags forces a > >>>btrfs_commit_transaction > >>>while holding subvol_sem. As a result, we have seen workloads where > >>>calling `btrfs property set -ts <subvol> ro false` hangs waiting for a > >>>legitimately slow commit. This gets even worse if the workload tries to > >>>set flags on multiple subvolumes and the ioctls pile up on subvol_sem. > >>> > >>>Change the commit to a btrfs_end_transaction so that the ioctl can > >>>return in a timely fashion and piggy back on a later commit. > >>> > >>>Signed-off-by: Boris Burkov <boris@bur.io> > >>>--- > >>> fs/btrfs/ioctl.c | 2 +- > >>> fs/btrfs/transaction.c | 4 ++-- > >>> 2 files changed, 3 insertions(+), 3 deletions(-) > >>> > >>>diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c > >>>index bd3511c5ca81..3ae484768ce7 100644 > >>>--- a/fs/btrfs/ioctl.c > >>>+++ b/fs/btrfs/ioctl.c > >>>@@ -1985,7 +1985,7 @@ static noinline int > >>>btrfs_ioctl_subvol_setflags(struct file *file, > >>> goto out_reset; > >>> } > >>> - ret = btrfs_commit_transaction(trans); > >>>+ ret = btrfs_end_transaction(trans); > >> > >>This means the setflag is not committed to disk, and if a powerloss > >>happens before a transaction commit, then the setflag operation just get > >>lost. > >> > >>This means, previously if this ioctl returns, users can expect that the > >>flag is always set no matter what, but now there is no guarantee. > >> > >>Personally I'm not sure if we really want that operation to be committed > >>to disk. > >>Maybe that transaction commit can be initialized in user space, so for > >>multiple setflags, we only commit once, thus saves a lot of time. > >> > > > >I'm of the opinion that we shouldn't be committing the transaction > >for stuff like this, unless there's a really good reason to. > >Especially given we're holding the subvol lock here, we should > >just do end_transaction. Thanks, > From a user perspective I'd appreciate having the option to set it > in a non-durable way (I have seen btrfs property sets hanging for a > long time as well). But currently my application kind of depends on > it being durable. Making it non-durable wouldn't break much and I > guess the old behaviour could be emulated by a "btrfs fi sync > <subvol>" afterwards, but idk how much other stuff depends on it > being durable. Making it consistent with btrfs subvol del with the > "-c" switch would be nice and consistent as well (and the -c switch > could be done via IOC_SYNC after setting the properties). Martin, Thanks for your perspective, that's helpful. Could you elaborate on how your application relies on the durability? I would just like to learn more about how this might affect people. I really like the -c idea, but I fear if people are broadly depending on that behavior by default, it wouldn't be enough.
On 07.08.2020 22:45 Boris Burkov wrote: > On Wed, Aug 05, 2020 at 01:40:16PM +0000, Martin Raiber wrote: >> On 05.08.2020 01:08 Josef Bacik wrote: >>> On 8/4/20 6:48 PM, Qu Wenruo wrote: >>>> >>>> On 2020/8/5 上午1:55, Boris Burkov wrote: >>>>> Currently, btrfs_ioctl_subvol_setflags forces a >>>>> btrfs_commit_transaction >>>>> while holding subvol_sem. As a result, we have seen workloads where >>>>> calling `btrfs property set -ts <subvol> ro false` hangs waiting for a >>>>> legitimately slow commit. This gets even worse if the workload tries to >>>>> set flags on multiple subvolumes and the ioctls pile up on subvol_sem. >>>>> >>>>> Change the commit to a btrfs_end_transaction so that the ioctl can >>>>> return in a timely fashion and piggy back on a later commit. >>>>> >>>>> Signed-off-by: Boris Burkov <boris@bur.io> >>>>> --- >>>>> fs/btrfs/ioctl.c | 2 +- >>>>> fs/btrfs/transaction.c | 4 ++-- >>>>> 2 files changed, 3 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c >>>>> index bd3511c5ca81..3ae484768ce7 100644 >>>>> --- a/fs/btrfs/ioctl.c >>>>> +++ b/fs/btrfs/ioctl.c >>>>> @@ -1985,7 +1985,7 @@ static noinline int >>>>> btrfs_ioctl_subvol_setflags(struct file *file, >>>>> goto out_reset; >>>>> } >>>>> - ret = btrfs_commit_transaction(trans); >>>>> + ret = btrfs_end_transaction(trans); >>>> This means the setflag is not committed to disk, and if a powerloss >>>> happens before a transaction commit, then the setflag operation just get >>>> lost. >>>> >>>> This means, previously if this ioctl returns, users can expect that the >>>> flag is always set no matter what, but now there is no guarantee. >>>> >>>> Personally I'm not sure if we really want that operation to be committed >>>> to disk. >>>> Maybe that transaction commit can be initialized in user space, so for >>>> multiple setflags, we only commit once, thus saves a lot of time. >>>> >>> I'm of the opinion that we shouldn't be committing the transaction >>> for stuff like this, unless there's a really good reason to. >>> Especially given we're holding the subvol lock here, we should >>> just do end_transaction. Thanks, >> From a user perspective I'd appreciate having the option to set it >> in a non-durable way (I have seen btrfs property sets hanging for a >> long time as well). But currently my application kind of depends on >> it being durable. Making it non-durable wouldn't break much and I >> guess the old behaviour could be emulated by a "btrfs fi sync >> <subvol>" afterwards, but idk how much other stuff depends on it >> being durable. Making it consistent with btrfs subvol del with the >> "-c" switch would be nice and consistent as well (and the -c switch >> could be done via IOC_SYNC after setting the properties). > Martin, > > Thanks for your perspective, that's helpful. Could you elaborate on how > your application relies on the durability? I would just like to learn > more about how this might affect people. > > I really like the -c idea, but I fear if people are broadly depending on > that behavior by default, it wouldn't be enough. It is a backup software that currently works a bit like this: 1. Add database entry for new backup A with done=0 2. Create btrfs subvol A for backup 3. rsync backup source to A 4. btrfs fi sync A 5. Set subvol A to read-only 6. Set database entry for A to done=1 On startup: Delete all btrfs subvols of backups where done!=1 in the database. Switching 4. and 5. should fix it if changing properties is not durable. Otherwise there could be subvols that don't get deleted on startup (after crash) and are not read-only. Those would be an annoyance e.g. if the backups are further replicated using btrfs end/receive, or if one relies on the finished backups being read-only. Worst case there is someone that leaves 4. out and relies on 5. to sync to disk (would that work?).
Mon, Aug 10, 2020 at 06:05:41PM +0000, Martin Raiber wrote: > On 07.08.2020 22:45 Boris Burkov wrote: > >On Wed, Aug 05, 2020 at 01:40:16PM +0000, Martin Raiber wrote: > >>On 05.08.2020 01:08 Josef Bacik wrote: > >>>On 8/4/20 6:48 PM, Qu Wenruo wrote: > >>>> > >>>>On 2020/8/5 上午1:55, Boris Burkov wrote: > >>>>>Currently, btrfs_ioctl_subvol_setflags forces a > >>>>>btrfs_commit_transaction > >>>>>while holding subvol_sem. As a result, we have seen workloads where > >>>>>calling `btrfs property set -ts <subvol> ro false` hangs waiting for a > >>>>>legitimately slow commit. This gets even worse if the workload tries to > >>>>>set flags on multiple subvolumes and the ioctls pile up on subvol_sem. > >>>>> > >>>>>Change the commit to a btrfs_end_transaction so that the ioctl can > >>>>>return in a timely fashion and piggy back on a later commit. > >>>>> > >>>>>Signed-off-by: Boris Burkov <boris@bur.io> > >>>>>--- > >>>>> fs/btrfs/ioctl.c | 2 +- > >>>>> fs/btrfs/transaction.c | 4 ++-- > >>>>> 2 files changed, 3 insertions(+), 3 deletions(-) > >>>>> > >>>>>diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c > >>>>>index bd3511c5ca81..3ae484768ce7 100644 > >>>>>--- a/fs/btrfs/ioctl.c > >>>>>+++ b/fs/btrfs/ioctl.c > >>>>>@@ -1985,7 +1985,7 @@ static noinline int > >>>>>btrfs_ioctl_subvol_setflags(struct file *file, > >>>>> goto out_reset; > >>>>> } > >>>>> - ret = btrfs_commit_transaction(trans); > >>>>>+ ret = btrfs_end_transaction(trans); > >>>>This means the setflag is not committed to disk, and if a powerloss > >>>>happens before a transaction commit, then the setflag operation just get > >>>>lost. > >>>> > >>>>This means, previously if this ioctl returns, users can expect that the > >>>>flag is always set no matter what, but now there is no guarantee. > >>>> > >>>>Personally I'm not sure if we really want that operation to be committed > >>>>to disk. > >>>>Maybe that transaction commit can be initialized in user space, so for > >>>>multiple setflags, we only commit once, thus saves a lot of time. > >>>> > >>>I'm of the opinion that we shouldn't be committing the transaction > >>>for stuff like this, unless there's a really good reason to. > >>>Especially given we're holding the subvol lock here, we should > >>>just do end_transaction. Thanks, > >> From a user perspective I'd appreciate having the option to set it > >>in a non-durable way (I have seen btrfs property sets hanging for a > >>long time as well). But currently my application kind of depends on > >>it being durable. Making it non-durable wouldn't break much and I > >>guess the old behaviour could be emulated by a "btrfs fi sync > >><subvol>" afterwards, but idk how much other stuff depends on it > >>being durable. Making it consistent with btrfs subvol del with the > >>"-c" switch would be nice and consistent as well (and the -c switch > >>could be done via IOC_SYNC after setting the properties). > >Martin, > > > >Thanks for your perspective, that's helpful. Could you elaborate on how > >your application relies on the durability? I would just like to learn > >more about how this might affect people. > > > >I really like the -c idea, but I fear if people are broadly depending on > >that behavior by default, it wouldn't be enough. > > It is a backup software that currently works a bit like this: > > 1. Add database entry for new backup A with done=0 > 2. Create btrfs subvol A for backup > 3. rsync backup source to A > 4. btrfs fi sync A > 5. Set subvol A to read-only > 6. Set database entry for A to done=1 > > On startup: Delete all btrfs subvols of backups where done!=1 in the > database. > > Switching 4. and 5. should fix it if changing properties is not > durable. Otherwise there could be subvols that don't get deleted on > startup (after crash) and are not read-only. Those would be an > annoyance e.g. if the backups are further replicated using btrfs > end/receive, or if one relies on the finished backups being > read-only. > > Worst case there is someone that leaves 4. out and relies on 5. to > sync to disk (would that work?). > > Thanks for the extra detail, that example makes sense. As I see it, our options to move forward are: 1. Leave the sync; suffer hung setflags calls, but no regression. 2a. Use this patch; risk affecting use-cases like Martin's. 2b. Also add a -c option to btrfs subvol setflags for people to move to. 3. Add an 'async' option to btrfs subvol setflags people can use if they're affected by the issue this patch fixes. 4. Introduce a new command for setting a subvol read only with a -c flag 1 is a bummer as it doesn't move us towards less unneeded syncs. 2a/b are "easy" but I fear they might be too risky. 3 and 4 introduce different kinds of ugliness into the user interface, but don't negatively affect existing use cases. I'm happy to write up whichever variant people think is best.
On 8/4/20 1:55 PM, Boris Burkov wrote: > Currently, btrfs_ioctl_subvol_setflags forces a btrfs_commit_transaction > while holding subvol_sem. As a result, we have seen workloads where > calling `btrfs property set -ts <subvol> ro false` hangs waiting for a > legitimately slow commit. This gets even worse if the workload tries to > set flags on multiple subvolumes and the ioctls pile up on subvol_sem. > > Change the commit to a btrfs_end_transaction so that the ioctl can > return in a timely fashion and piggy back on a later commit. > > Signed-off-by: Boris Burkov <boris@bur.io> Reviewed-by: Josef Bacik <josef@toxicpanda.com> I think we follow up with a btrfs-progs patch to make syncing an option with setflags (or hell do it by default and make the option to not sync). Having the commit here was arbitrary and not needed. Thanks, Josef
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index bd3511c5ca81..3ae484768ce7 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -1985,7 +1985,7 @@ static noinline int btrfs_ioctl_subvol_setflags(struct file *file, goto out_reset; } - ret = btrfs_commit_transaction(trans); + ret = btrfs_end_transaction(trans); out_reset: if (ret) diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 20c6ac1a5de7..1dc44209c2ae 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -47,7 +47,7 @@ * | Will wait for previous running transaction to completely finish if there * | is one * | - * | Then one of the following happes: + * | Then one of the following happens: * | - Wait for all other trans handle holders to release. * | The btrfs_commit_transaction() caller will do the commit work. * | - Wait for current transaction to be committed by others. @@ -60,7 +60,7 @@ * | * | To next stage: * | Caller is chosen to commit transaction N, and all other trans handle - * | haven been released. + * | have been released. * V * Transaction N [[TRANS_STATE_COMMIT_DOING]] * |
Currently, btrfs_ioctl_subvol_setflags forces a btrfs_commit_transaction while holding subvol_sem. As a result, we have seen workloads where calling `btrfs property set -ts <subvol> ro false` hangs waiting for a legitimately slow commit. This gets even worse if the workload tries to set flags on multiple subvolumes and the ioctls pile up on subvol_sem. Change the commit to a btrfs_end_transaction so that the ioctl can return in a timely fashion and piggy back on a later commit. Signed-off-by: Boris Burkov <boris@bur.io> --- fs/btrfs/ioctl.c | 2 +- fs/btrfs/transaction.c | 4 ++-- 2 files changed, 3 insertions(+), 3 deletions(-)