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From: Miklos Szeredi <miklos@szeredi.hu>
Date: Sun, 5 Feb 2017 22:19:20 +0100
Message-ID: <CAJfpeguzOgX9d+5XCNJcmXW5KkrGbnWB5aZSP1-0q3a6i6uk2w@mail.gmail.com>
Subject: Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to
 allocate more pages per call
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linux NFS list <linux-nfs@vger.kernel.org>, ceph-devel@vger.kernel.org,
        lustre-devel@lists.lustre.org, v9fs-developer@lists.sourceforge.net,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Chris Wilson <chris@chris-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Jeff Layton <jlayton@redhat.com>
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Sender: linux-nfs-owner@vger.kernel.org

On Sun, Feb 5, 2017 at 10:01 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sun, Feb 05, 2017 at 09:15:24PM +0100, Miklos Szeredi wrote:
>
>> That case is fine.   But nothing guarantees that fuse_abort_conn()
>> won't be called (in the non-deadlock case) when data is being copied
>> to the request args.  Ending the request at such a point could easily
>> lead to use after free,
>
> So why not leave ending it to your fuse_dev_do_write()/fuse_dev_do_read()?
> See the reply I'd just sent (your mail arrived while I'd been writing that
> one - saw it only after I'd sent mine).
>
> Basically, what if we keep FR_LOCKED through *all* fuse_dev_do_{read,write}(),
> rather than dropping and regaining it many times and have fuse_abort_conn()
> skip request_end() on FR_LOCKED ones?

Then we can't break out of that deadlock: we wait until
fuse_dev_do_write() is done until calling request_end() which
ultimately results in unlocking page.  But fuse_dev_do_write() won't
complete until the page is unlocked.

The only way out that I see is to have a refcount on all pages in
args.  Which means copying everything not already in refcountable page
(i.e. args on stack) to a page array.   It's definitely doable, but
needs time to sort out, and I'm definitely lacking that (overlayfs
currently trumps fuse).

Thanks,
Miklos