linux-kernel - Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator

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Message-ID: <Y9MgVsrMdgGsxNHC@ZenIV>
Date:   Fri, 27 Jan 2023 00:52:38 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list
 from an iterator

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).

Said that, I still think that pinned pages (arbitrary pagecache ones,
at that) ending up in a pipe is a seriously bad idea.  It's trivial to
arrange for them to stay that way indefinitely - no priveleges needed,
very few limits, etc.