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From: Kirill Smelkov <kirr@nexedi.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Miklos Szeredi <mszeredi@redhat.com>,
	<linux-fsdevel@vger.kernel.org>,
	fuse-devel <fuse-devel@lists.sourceforge.net>,
	Han-Wen Nienhuys <hanwen@google.com>,
	Jakob Unterwurzacher <jakobunt@gmail.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [RESEND, PATCH v2] fuse: Don't drop NOTIFY_REPLY if we promised it
Date: Thu, 28 Feb 2019 11:48:03 +0000	[thread overview]
Message-ID: <20190228114757.GA2796@deco.navytux.spb.ru> (raw)
In-Reply-To: <CAJfpegvXK+AmpWFSF1qdVt+EFTBwU94AD_JZCEeFF1KEZCObwA@mail.gmail.com>

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@nexedi.com> 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@nexedi.com>
Date: Thu, 28 Feb 2019 13:06:18 +0300
Subject: [PATCH 1/2] fuse: retrieve: cap requested size to negotiated
 max_write
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit

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>
Cc: <stable@vger.kernel.org> # 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


  reply	other threads:[~2019-02-28 12:03 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-19  9:42 [RESEND, PATCH v2] fuse: Don't drop NOTIFY_REPLY if we promised it Kirill Smelkov
2019-02-26 15:14 ` Miklos Szeredi
2019-02-27 20:02   ` Kirill Smelkov
2019-02-27 20:26     ` Miklos Szeredi
2019-02-27 20:39       ` Kirill Smelkov
2019-02-28  8:10         ` Miklos Szeredi
2019-02-28 11:48           ` Kirill Smelkov [this message]
2019-02-28 11:50             ` [PATCH 2/2] fuse: require /dev/fuse reads to have enough buffer capacity as negotiated Kirill Smelkov
2019-03-07  9:34             ` [RESEND, PATCH v2] fuse: Don't drop NOTIFY_REPLY if we promised it Kirill Smelkov
2019-03-14 10:45               ` [RESEND3, PATCH 0/2] fuse: don't stuck clients on retrieve_notify with size > max_write Kirill Smelkov
2019-03-14 10:46                 ` [PATCH 1/2] fuse: retrieve: cap requested size to negotiated max_write Kirill Smelkov
2019-03-14 10:46                 ` [PATCH 2/2] fuse: require /dev/fuse reads to have enough buffer capacity as negotiated Kirill Smelkov

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