linux-kernel - Re: [PATCH 1/2] fuse: postpone end_page_writeback() in fuse_writepage

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Message-ID: <CAJfpegvQVoc2x-2tr9vPQdj-eF9mcgCe8rQ=MFBi6ErMLgkEqA@mail.gmail.com>
Date:	Thu, 29 Aug 2013 18:27:11 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Maxim Patlasov <MPatlasov@...allels.com>
Cc:	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Brian Foster <bfoster@...hat.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org, Pavel Emelianov <xemul@...allels.com>
Subject: Re: [PATCH 1/2] fuse: postpone end_page_writeback() in fuse_writepage_locked()

On Mon, Aug 12, 2013 at 6:39 PM, Maxim Patlasov <MPatlasov@...allels.com> wrote:
> The patch fixes a race between ftruncate(2), mmap-ed write and write(2):
>
> 1) An user makes a page dirty via mmap-ed write.
> 2) The user performs shrinking truncate(2) intended to purge the page.
> 3) Before fuse_do_setattr calls truncate_pagecache, the page goes to
>    writeback. fuse_writepage_locked fills FUSE_WRITE request and releases
>    the original page by end_page_writeback.
> 4) fuse_do_setattr() completes and successfully returns. Since now, i_mutex
>    is free.
> 5) Ordinary write(2) extends i_size back to cover the page. Note that
>    fuse_send_write_pages do wait for fuse writeback, but for another
>    page->index.
> 6) fuse_writepage_locked proceeds by queueing FUSE_WRITE request.
>    fuse_send_writepage is supposed to crop inarg->size of the request,
>    but it doesn't because i_size has already been extended back.
>
> Moving end_page_writeback to the end of fuse_writepage_locked fixes the race
> because now the fact that truncate_pagecache is successfully returned infers
> that fuse_writepage_locked has already called end_page_writeback. And this,
> in turn, infers that fuse_flush_writepages has already called
> fuse_send_writepage, and the latter used valid (shrunk) i_size. write(2)
> could not extend it because of i_mutex held by ftruncate(2).
>
> Signed-off-by: Maxim Patlasov <mpatlasov@...allels.com>


Applied this, too.

Thanks,
Miklos

> ---
>  fs/fuse/file.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 5c121fe..d1715b3 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1529,7 +1529,6 @@ static int fuse_writepage_locked(struct page *page)
>
>         inc_bdi_stat(mapping->backing_dev_info, BDI_WRITEBACK);
>         inc_zone_page_state(tmp_page, NR_WRITEBACK_TEMP);
> -       end_page_writeback(page);
>
>         spin_lock(&fc->lock);
>         list_add(&req->writepages_entry, &fi->writepages);
> @@ -1537,6 +1536,8 @@ static int fuse_writepage_locked(struct page *page)
>         fuse_flush_writepages(inode);
>         spin_unlock(&fc->lock);
>
> +       end_page_writeback(page);
> +
>         return 0;
>
>  err_free:
>
--
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