| Message ID | 20180528091923.GH1517@dhcp22.suse.cz (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, May 28, 2018 at 11:19:23AM +0200, Michal Hocko wrote: > On Mon 28-05-18 09:48:54, Dave Chinner wrote: > > On Fri, May 25, 2018 at 10:16:24AM +0200, Michal Hocko wrote: > > > On Fri 25-05-18 08:17:15, Dave Chinner wrote: > > > > On Thu, May 24, 2018 at 01:43:41PM +0200, Michal Hocko wrote: > > > [...] > > > > > +FS/IO code then simply calls the appropriate save function right at the > > > > > +layer where a lock taken from the reclaim context (e.g. shrinker) and > > > > > +the corresponding restore function when the lock is released. All that > > > > > +ideally along with an explanation what is the reclaim context for easier > > > > > +maintenance. > > > > > > > > This paragraph doesn't make much sense to me. I think you're trying > > > > to say that we should call the appropriate save function "before > > > > locks are taken that a reclaim context (e.g a shrinker) might > > > > require access to." > > > > > > > > I think it's also worth making a note about recursive/nested > > > > save/restore stacking, because it's not clear from this description > > > > that this is allowed and will work as long as inner save/restore > > > > calls are fully nested inside outer save/restore contexts. > > > > > > Any better? > > > > > > -FS/IO code then simply calls the appropriate save function right at the > > > -layer where a lock taken from the reclaim context (e.g. shrinker) and > > > -the corresponding restore function when the lock is released. All that > > > -ideally along with an explanation what is the reclaim context for easier > > > -maintenance. > > > +FS/IO code then simply calls the appropriate save function before any > > > +lock shared with the reclaim context is taken. The corresponding > > > +restore function when the lock is released. All that ideally along with > > > +an explanation what is the reclaim context for easier maintenance. > > > + > > > +Please note that the proper pairing of save/restore function allows nesting > > > +so memalloc_noio_save is safe to be called from an existing NOIO or NOFS scope. > > > > It's better, but the talk of this being necessary for locking makes > > me cringe. XFS doesn't do it for locking reasons - it does it > > largely for preventing transaction context nesting, which has all > > sorts of problems that cause hangs (e.g. log space reservations > > can't be filled) that aren't directly locking related. > > Yeah, I wanted to not mention locks as much as possible. > > > i.e we should be talking about using these functions around contexts > > where recursion back into the filesystem through reclaim is > > problematic, not that "holding locks" is problematic. Locks can be > > used as an example of a problematic context, but locks are not the > > only recursion issue that require GFP_NOFS allocation contexts to > > avoid. > > agreed. Do you have any suggestion how to add a more abstract wording > that would not make head spinning? > > I've tried the following. Any better? > > diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst > index c0ec212d6773..adac362b2875 100644 > --- a/Documentation/core-api/gfp_mask-from-fs-io.rst > +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst > @@ -34,9 +34,11 @@ scope will inherently drop __GFP_FS respectively __GFP_IO from the given > mask so no memory allocation can recurse back in the FS/IO. > > FS/IO code then simply calls the appropriate save function before any > -lock shared with the reclaim context is taken. The corresponding > -restore function when the lock is released. All that ideally along with > -an explanation what is the reclaim context for easier maintenance. > +critical section wrt. the reclaim is started - e.g. lock shared with the > +reclaim context or when a transaction context nesting would be possible > +via reclaim. The corresponding restore function when the critical .... restore function should be called when ... But otherwise I think this is much better. Cheers, Dave.
On Tue 29-05-18 08:32:05, Dave Chinner wrote: > On Mon, May 28, 2018 at 11:19:23AM +0200, Michal Hocko wrote: > > On Mon 28-05-18 09:48:54, Dave Chinner wrote: > > > On Fri, May 25, 2018 at 10:16:24AM +0200, Michal Hocko wrote: > > > > On Fri 25-05-18 08:17:15, Dave Chinner wrote: > > > > > On Thu, May 24, 2018 at 01:43:41PM +0200, Michal Hocko wrote: > > > > [...] > > > > > > +FS/IO code then simply calls the appropriate save function right at the > > > > > > +layer where a lock taken from the reclaim context (e.g. shrinker) and > > > > > > +the corresponding restore function when the lock is released. All that > > > > > > +ideally along with an explanation what is the reclaim context for easier > > > > > > +maintenance. > > > > > > > > > > This paragraph doesn't make much sense to me. I think you're trying > > > > > to say that we should call the appropriate save function "before > > > > > locks are taken that a reclaim context (e.g a shrinker) might > > > > > require access to." > > > > > > > > > > I think it's also worth making a note about recursive/nested > > > > > save/restore stacking, because it's not clear from this description > > > > > that this is allowed and will work as long as inner save/restore > > > > > calls are fully nested inside outer save/restore contexts. > > > > > > > > Any better? > > > > > > > > -FS/IO code then simply calls the appropriate save function right at the > > > > -layer where a lock taken from the reclaim context (e.g. shrinker) and > > > > -the corresponding restore function when the lock is released. All that > > > > -ideally along with an explanation what is the reclaim context for easier > > > > -maintenance. > > > > +FS/IO code then simply calls the appropriate save function before any > > > > +lock shared with the reclaim context is taken. The corresponding > > > > +restore function when the lock is released. All that ideally along with > > > > +an explanation what is the reclaim context for easier maintenance. > > > > + > > > > +Please note that the proper pairing of save/restore function allows nesting > > > > +so memalloc_noio_save is safe to be called from an existing NOIO or NOFS scope. > > > > > > It's better, but the talk of this being necessary for locking makes > > > me cringe. XFS doesn't do it for locking reasons - it does it > > > largely for preventing transaction context nesting, which has all > > > sorts of problems that cause hangs (e.g. log space reservations > > > can't be filled) that aren't directly locking related. > > > > Yeah, I wanted to not mention locks as much as possible. > > > > > i.e we should be talking about using these functions around contexts > > > where recursion back into the filesystem through reclaim is > > > problematic, not that "holding locks" is problematic. Locks can be > > > used as an example of a problematic context, but locks are not the > > > only recursion issue that require GFP_NOFS allocation contexts to > > > avoid. > > > > agreed. Do you have any suggestion how to add a more abstract wording > > that would not make head spinning? > > > > I've tried the following. Any better? > > > > diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst > > index c0ec212d6773..adac362b2875 100644 > > --- a/Documentation/core-api/gfp_mask-from-fs-io.rst > > +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst > > @@ -34,9 +34,11 @@ scope will inherently drop __GFP_FS respectively __GFP_IO from the given > > mask so no memory allocation can recurse back in the FS/IO. > > > > FS/IO code then simply calls the appropriate save function before any > > -lock shared with the reclaim context is taken. The corresponding > > -restore function when the lock is released. All that ideally along with > > -an explanation what is the reclaim context for easier maintenance. > > +critical section wrt. the reclaim is started - e.g. lock shared with the > > +reclaim context or when a transaction context nesting would be possible > > +via reclaim. The corresponding restore function when the critical > > .... restore function should be called when ... fixed > But otherwise I think this is much better. Thanks!
diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst index c0ec212d6773..adac362b2875 100644 --- a/Documentation/core-api/gfp_mask-from-fs-io.rst +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst @@ -34,9 +34,11 @@ scope will inherently drop __GFP_FS respectively __GFP_IO from the given mask so no memory allocation can recurse back in the FS/IO. FS/IO code then simply calls the appropriate save function before any -lock shared with the reclaim context is taken. The corresponding -restore function when the lock is released. All that ideally along with -an explanation what is the reclaim context for easier maintenance. +critical section wrt. the reclaim is started - e.g. lock shared with the +reclaim context or when a transaction context nesting would be possible +via reclaim. The corresponding restore function when the critical +section ends. All that ideally along with an explanation what is +the reclaim context for easier maintenance. Please note that the proper pairing of save/restore function allows nesting so memalloc_noio_save is safe to be called from an existing NOIO or NOFS scope.