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    SubjectRe: [PATCH] doc: document scope NOFS, NOIO APIs
    On Thu, May 24, 2018 at 01:43:41PM +0200, Michal Hocko wrote:
    > From: Michal Hocko <mhocko@suse.com>
    >
    > 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@oracle.com>
    > Cc: David Sterba <dsterba@suse.cz>
    > Requested-by: "Theodore Y. Ts'o" <tytso@mit.edu>
    > Signed-off-by: Michal Hocko <mhocko@suse.com>

    Yay, Documentation! :)

    > ---
    >
    > 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@thunk.org
    > .../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@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
    > +resp. __GFP_IO (note the later implies clearing the first as well) in
    > +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
    > +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.

    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.

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com

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