Re: [RESEND, PATCH v2] fuse: Don't drop NOTIFY_REPLY if we promised it

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On Thu, Feb 28, 2019 at 09:10:15AM +0100, Miklos Szeredi wrote:
> On Wed, Feb 27, 2019 at 9:39 PM Kirill Smelkov <kirr@xxxxxxxxxx> wrote:
> 
> > I more or less agree with this statement. However can we please make the
> > breakage to be explicitly visible with an error instead of exhibiting it
> > via harder to debug stucks/deadlocks? For example sys_read < max_write
> > -> error instead of getting stuck. And if notify_retrieve requests
> > buffer larger than max_write -> error or cut to max_write, but don't
> > return OK when we know we will never send what was requested to
> > filesystem even if it uses max_write sized reads. What is the point of
> > breaking in hard to diagnose way when we can make the breakage showing
> > itself explicitly? Would a patch for such behaviour accepted?
> 
> Sure, if it's only adds a couple of lines.   Adding more than say ten
> lines for such a non-bug fix is definitely excessive.

Ok, thanks. Please consider applying the following patch. (It's a bit
pity to hear the problem is not considered to be a bug, but anyway).

I will also send the second patch as another mail, since I could not
made `git am --scissors` to apply several patched extracted from one
mail successfully.

Thanks,
Kirill

---- 8< ----
From: Kirill Smelkov <kirr@xxxxxxxxxx>
Date: Thu, 28 Feb 2019 13:06:18 +0300
Subject: [PATCH 1/2] fuse: retrieve: cap requested size to negotiated
 max_write
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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@xxxxxxxxxx>
Cc: Han-Wen Nienhuys <hanwen@xxxxxxxxxx>
Cc: Jakob Unterwurzacher <jakobunt@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v2.6.36+
---
 fs/fuse/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 8a63e52785e9..38e94bc43053 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -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)
-- 
2.21.0.352.gf09ad66450





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