Date:   Fri, 27 Jan 2023 01:21:26 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     David Howells <dhowells@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Christoph Hellwig <hch@lst.de>,
        John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list
 from an iterator
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On Fri, Jan 27, 2023 at 12:52:38AM +0000, Al Viro wrote:
> On Thu, Jan 26, 2023 at 11:56:50PM +0000, David Howells wrote:
> > Al says that pinning a page (ie. FOLL_PIN) could cause a deadlock if a page is
> > vmspliced into a pipe with the pipe holding a pin on it because pinned pages
> > are removed from all page tables.  Is this actually the case?  I can't see
> > offhand where in mm/gup.c it does this.
> 
> It doesn't; sorry, really confused memories of what's going on, took a while
> to sort them out (FWIW, writeback is where we unmap and check if page is
> pinned, while pin_user_pages running into an unmapped page will end up
> with handle_mm_fault() (->fault(), actually) try to get the sucker locked
> and block on that until the writeback is over).

Umm...  OK, I really need to reread that area.  Hopefully will be done with
that by tomorrow...