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: <82368bae-b2cc-5d55-6de6-e3283cef063b@redhat.com>
Date:   Tue, 2 May 2023 21:05:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
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Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

On 02.05.23 20:59, Peter Zijlstra wrote:
> 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.

I completely agree, and I think we should fix that in 
page_is_secretmem() as well.

-- 
Thanks,

David / dhildenb