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fuse: retrieve: cap requested size to negotiated max_write 

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
back.

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-fuse

Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
Cc: Han-Wen Nienhuys <hanwen@google.com>
Cc: Jakob Unterwurzacher <jakobunt@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Kirill Smelkov 2019-03-27 10:15:19 +00:00 committed by Miklos Szeredi
parent ad2ba64dd4
commit 7640682e67
1 changed files with 1 additions and 1 deletions
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2
fs/fuse/dev.c
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@ -1749,7 +1749,7 @@ static int fuse_retrieve(struct fuse_conn *fc, struct inode *inode,
offset = outarg->offset & ~PAGE_MASK;
file_size = i_size_read(inode);
num = outarg->size;
num = min(outarg->size, fc->max_write);
if (outarg->offset > file_size)
num = 0;
else if (outarg->offset + num > file_size)