linux-kernel - Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <20230502185926.GE4253@hirez.programming.kicks-ass.net>
Date:   Tue, 2 May 2023 20:59:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 07:34:06PM +0200, David Hildenbrand wrote:
> Now, if we read folio->mapping after checking if the page we pinned is still
> mapped (PTE unchanged), at least the page we pinned cannot be reused in the
> meantime. I suspect that we can still read "NULL" on the second read. But
> whatever we dereference from the first read should still be valid, even if
> the second read would have returned NULL ("rcu freeing").

Right, but given it's the compiler adding loads we're not sure what if
anything it uses and it gets very hard to reason about the code.

This is where READ_ONCE() helps, we instruct the compiler to only do a
single load and we can still reason about the code.