On Thu 24-05-18 14:52:02, Jonathan Corbet wrote: > On Thu, 24 May 2018 13:43:41 +0200 > Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > Although the api is documented in the source code Ted has pointed out > > that there is no mention in the core-api Documentation and there are > > people looking there to find answers how to use a specific API. > > > > Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx> > > Cc: David Sterba <dsterba@xxxxxxx> > > Requested-by: "Theodore Y. Ts'o" <tytso@xxxxxxx> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > --- > > > > Hi Johnatan, > > Ted has proposed this at LSFMM and then we discussed that briefly on the > > mailing list [1]. I received some useful feedback from Darrick and Dave > > which has been (hopefully) integrated. Then the thing fall off my radar > > rediscovering it now when doing some cleanup. Could you take the patch > > please? > > > > [1] http://lkml.kernel.org/r/20180424183536.GF30619@xxxxxxxxx > > .../core-api/gfp_mask-from-fs-io.rst | 55 +++++++++++++++++++ > > 1 file changed, 55 insertions(+) > > create mode 100644 Documentation/core-api/gfp_mask-from-fs-io.rst > > So you create the rst file, but don't add it in index.rst; that means it > won't be a part of the docs build and Sphinx will complain. I am not really familiar with how the whole rst thing works. diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index c670a8031786..8a5f48ef16f2 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -25,6 +25,7 @@ Core utilities genalloc errseq printk-formats + gfp_mask-from-fs-io Interfaces for kernel debugging =============================== This? > > > diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst > > new file mode 100644 > > index 000000000000..e8b2678e959b > > --- /dev/null > > +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst > > @@ -0,0 +1,55 @@ > > +================================= > > +GFP masks used from FS/IO context > > +================================= > > + > > +:Date: Mapy, 2018 > > Ah...the wonderful month of Mapy....:) fixed > > +:Author: Michal Hocko <mhocko@xxxxxxxxxx> > > + > > +Introduction > > +============ > > + > > +Code paths in the filesystem and IO stacks must be careful when > > +allocating memory to prevent recursion deadlocks caused by direct > > +memory reclaim calling back into the FS or IO paths and blocking on > > +already held resources (e.g. locks - most commonly those used for the > > +transaction context). > > + > > +The traditional way to avoid this deadlock problem is to clear __GFP_FS > > +resp. __GFP_IO (note the later implies clearing the first as well) in > > "resp." is indeed a bit terse. Even spelled out as "respectively", though, OK s@resp\.@respectively@g > I'm not sure what the word is intended to mean here. Did you mean "or"? Basically yes. There are two cases here. NOFS and NOIO. The later being a subset of the first. I didn't really want to repeat the whole thing for NOIO. > > > +the gfp mask when calling an allocator. GFP_NOFS resp. GFP_NOIO can be > > Here too. > > > +used as shortcut. It turned out though that above approach has led to > > +abuses when the restricted gfp mask is used "just in case" without a > > +deeper consideration which leads to problems because an excessive use > > +of GFP_NOFS/GFP_NOIO can lead to memory over-reclaim or other memory > > +reclaim issues. > > + > > +New API > > +======== > > + > > +Since 4.12 we do have a generic scope API for both NOFS and NOIO context > > +``memalloc_nofs_save``, ``memalloc_nofs_restore`` resp. ``memalloc_noio_save``, > > +``memalloc_noio_restore`` which allow to mark a scope to be a critical > > +section from the memory reclaim recursion into FS/IO POV. Any allocation > > "from a filesystem or I/O point of view" ? OK > > +from that scope will inherently drop __GFP_FS resp. __GFP_IO from the given > > +mask so no memory allocation can recurse back in the FS/IO. > > Wouldn't it be nice if those functions had kerneldoc comments that could be > pulled in here! :) Most probably yes ;) I thought I've done that but that was probably in a different universe. This probably? diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index e1f8411e6b80..f49ece8ee37a 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -166,6 +166,17 @@ static inline void fs_reclaim_acquire(gfp_t gfp_mask) { } static inline void fs_reclaim_release(gfp_t gfp_mask) { } #endif +/** + * memalloc_noio_save - Marks implicit GFP_NOIO allocation scope. + * + * This functions marks the beginning of the GFP_NOIO allocation scope. + * All further allocations will implicitly drop __GFP_IO flag and so + * they are safe for the IO critical section from the allocation recursion + * point of view. Use memalloc_noio_restore to end the scope with flags + * returned by this function. + * + * This function is safe to be used from any context. + */ static inline unsigned int memalloc_noio_save(void) { unsigned int flags = current->flags & PF_MEMALLOC_NOIO; @@ -173,11 +184,30 @@ static inline unsigned int memalloc_noio_save(void) return flags; } +/** + * memalloc_noio_restore - Ends the implicit GFP_NOIO scope. + * @flags: Flags to restore. + * + * Ends the implicit GFP_NOIO scope started by memalloc_noio_save function. + * Always make sure that that the given flags is the return value from the + * pairing memalloc_noio_save call. + */ static inline void memalloc_noio_restore(unsigned int flags) { current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags; } +/** + * memalloc_nofs_save - Marks implicit GFP_NOFS allocation scope. + * + * This functions marks the beginning of the GFP_NOFS allocation scope. + * All further allocations will implicitly drop __GFP_FS flag and so + * they are safe for the FS critical section from the allocation recursion + * point of view. Use memalloc_nofs_restore to end the scope with flags + * returned by this function. + * + * This function is safe to be used from any context. + */ static inline unsigned int memalloc_nofs_save(void) { unsigned int flags = current->flags & PF_MEMALLOC_NOFS; @@ -185,6 +215,14 @@ static inline unsigned int memalloc_nofs_save(void) return flags; } +/** + * memalloc_nofs_restore - Ends the implicit GFP_NOFS scope. + * @flags: Flags to restore. + * + * Ends the implicit GFP_NOFS scope started by memalloc_nofs_save function. + * Always make sure that that the given flags is the return value from the + * pairing memalloc_nofs_save call. + */ static inline void memalloc_nofs_restore(unsigned int flags) { current->flags = (current->flags & ~PF_MEMALLOC_NOFS) | flags; > > +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 > > where a lock *is* taken ? fixed -- Michal Hocko SUSE Labs
