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

[Miklos Szeredi]

On Wed, Nov 24, 2021 at 1:40 AM Justin Forbes <jmforbes@xxxxxxxxxxx> 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
From: Miklos Szeredi <mszeredi@xxxxxxxxxx>
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@xxxxxxxxxxx>
Fixes: 712a951025c0 ("fuse: fix page stealing")
Cc: <stable@xxxxxxxxxxxxxxx> # v2.6.35
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 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))