• Kirill Smelkov's avatar
    fuse: retrieve: cap requested size to negotiated max_write · 1e0a2528
    Kirill Smelkov authored
    [ Upstream commit 7640682e67b33cab8628729afec8ca92b851394f ]
    FUSE filesystem server and kernel client negotiate during initialization
    phase, what should be the maximum write size the client will ever issue.
    Correspondingly the filesystem server then queues sys_read calls to read
    requests with buffer capacity large enough to carry request header + that
    max_write bytes. A filesystem server is free to set its max_write in
    anywhere in the range between [1*page, fc->max_pages*page]. In particular
    go-fuse[2] sets max_write by default as 64K, wheres default fc->max_pages
    corresponds to 128K. Libfuse also allows users to configure max_write, but
    by default presets it to possible maximum.
    If max_write is < fc->max_pages*page, and in NOTIFY_RETRIEVE handler we
    allow to retrieve more than max_write bytes, corresponding prepared
    NOTIFY_REPLY will be thrown away by fuse_dev_do_read, because the
    filesystem server, in full correspondence with server/client contract, will
    be only queuing sys_read with ~max_write buffer capacity, and
    fuse_dev_do_read throws away requests that cannot fit into server request
    buffer. In turn the filesystem server could get stuck waiting indefinitely
    for NOTIFY_REPLY since NOTIFY_RETRIEVE handler returned OK which is
    understood by clients as that NOTIFY_REPLY was queued and will be sent
    Cap requested size to negotiate max_write to avoid the problem.  This
    aligns with the way NOTIFY_RETRIEVE handler works, which already
    unconditionally caps requested retrieve size to fuse_conn->max_pages.  This
    way it should not hurt NOTIFY_RETRIEVE semantic if we return less data than
    was originally requested.
    Please see [1] for context where the problem of stuck filesystem was hit
    for real, how the situation was traced and for more involving patch that
    did not make it into the tree.
    [1] https://marc.info/?l=linux-fsdevel&m=155057023600853&w=2
    [2] https://github.com/hanwen/go-fuseSigned-off-by: default avatarKirill Smelkov <kirr@nexedi.com>
    Cc: Han-Wen Nienhuys <hanwen@google.com>
    Cc: Jakob Unterwurzacher <jakobunt@gmail.com>
    Signed-off-by: default avatarMiklos Szeredi <mszeredi@redhat.com>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
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