Re: [PATCH 5.15 056/917] fuse: fix page stealing

Miklos Szeredi
2021-11-24T09:26:09Z

[-- Attachment #1: Type: text/plain, Size: 562 bytes --]

On Wed, Nov 24, 2021 at 1:40 AM Justin Forbes <jmforbes@linuxtx.org> wrote:
> Thanks, did a scratch build for that and dropped it in the bug. Only
> one user has reported back, but the report was that it did not fix the
> issue. I have also gotten confirmation now that the issue is occuring
> with 5.16-rc2.

Okay.

Morning light brings clarity to the mind. Here's a patch that should
definitely fix this bug, as well as the very unlikely race of the page
being truncated from the page cache before pipe_buf_release() is
called.

Please test.

Thanks,
Miklos

[-- Attachment #2: fuse-release-pipe-buf-after-its-last-use.patch --]
[-- Type: text/x-patch, Size: 1281 bytes --]

From: Miklos Szeredi <mszeredi@redhat.com>
Subject: fuse: release pipe buf after last use

Checking buf->flags should be done before the pipe_buf_release() is called
on the pipe buffer, since releasing the buffer might modify the flags.

This is exactly what page_cache_pipe_buf_release() does, and which results
in the same VM_BUG_ON_PAGE(PageLRU(page)) that the original patch was
trying to fix.

Reported-by: Justin Forbes <jmforbes@linuxtx.org>
Fixes: 712a951025c0 ("fuse: fix page stealing")
Cc: <stable@vger.kernel.org> # v2.6.35
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
fs/fuse/dev.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -847,17 +847,17 @@ static int fuse_try_move_page(struct fus

replace_page_cache_page(oldpage, newpage);

+ get_page(newpage);
+
+ if (!(buf->flags & PIPE_BUF_FLAG_LRU))
+ lru_cache_add(newpage);
+
/*
* Release while we have extra ref on stolen page. Otherwise
* anon_pipe_buf_release() might think the page can be reused.
*/
pipe_buf_release(cs->pipe, buf);

- get_page(newpage);
-
- if (!(buf->flags & PIPE_BUF_FLAG_LRU))
- lru_cache_add(newpage);
-
err = 0;
spin_lock(&cs->req->waitq.lock);
if (test_bit(FR_ABORTED, &cs->req->flags))