doc: document scope NOFS, NOIO APIs

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.

Requested-by: "Theodore Y. Ts'o" <tytso@mit.edu>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 61 insertions, 0 deletions

diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
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+=================================
+GFP masks used from FS/IO context
+=================================
+
+:Date: May, 2018
+:Author: Michal Hocko <mhocko@kernel.org>
+
+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
+respectively __GFP_IO (note the latter implies clearing the first as well) in
+the gfp mask when calling an allocator. GFP_NOFS respectively 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`` respectively ``memalloc_noio_save``,
+``memalloc_noio_restore`` which allow to mark a scope to be a critical
+section from a filesystem or I/O point of view. Any allocation from that
+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 critical section with respect to the reclaim is started - e.g.
+lock shared with the reclaim context or when a transaction context
+nesting would be possible via reclaim. The restore function should be
+called 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 functions
+allows nesting so it is safe to call ``memalloc_noio_save`` or
+``memalloc_noio_restore`` respectively from an existing NOIO or NOFS
+scope.
+
+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.
+
+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.