On 05/24/2018 04:43 AM, Michal Hocko 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 > > 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 > +: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 latter > +the gfp mask when calling an allocator. GFP_NOFS resp. GFP_NOIO can be > +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 s/POV/point of view/ or whatever it is. > +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. > + > +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. > + > +What about __vmalloc(GFP_NOFS) > +============================== > + > +vmalloc doesn't support GFP_NOFS semantic because there are hardcoded > +GFP_KERNEL allocations deep inside the allocator which are quite non-trivial > +to fix up. That means that calling ``vmalloc`` with GFP_NOFS/GFP_NOIO is > +almost always a bug. The good news is that the NOFS/NOIO semantic can be > +achieved by the scope api. I would prefer s/api/API/ throughout. > + > +In the ideal world, upper layers should already mark dangerous contexts > +and so no special care is required and vmalloc should be called without > +any problems. Sometimes if the context is not really clear or there are > +layering violations then the recommended way around that is to wrap ``vmalloc`` > +by the scope API with a comment explaining the problem. > -- ~Randy
