From: David Hildenbrand <david@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Jason Gunthorpe <jgg@ziepe.ca>, Jens Axboe <axboe@kernel.dk>,
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Mike Rapoport <rppt@linux.ibm.com>
Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings
Date: Tue, 2 May 2023 21:05:25 +0200 [thread overview]
Message-ID: <82368bae-b2cc-5d55-6de6-e3283cef063b@redhat.com> (raw)
In-Reply-To: <20230502185926.GE4253@hirez.programming.kicks-ass.net>
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
next prev parent reply other threads:[~2023-05-02 19:06 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-02 16:34 [PATCH v7 0/3] mm/gup: disallow GUP writing to file-backed mappings by default Lorenzo Stoakes
2023-05-02 16:34 ` [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic Lorenzo Stoakes
2023-05-02 16:38 ` David Hildenbrand
2023-05-02 16:53 ` Lorenzo Stoakes
2023-05-02 17:09 ` Lorenzo Stoakes
2023-05-02 17:16 ` David Hildenbrand
2023-05-02 16:34 ` [PATCH v7 2/3] mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings Lorenzo Stoakes
2023-05-02 16:42 ` David Hildenbrand
2023-05-02 16:56 ` Lorenzo Stoakes
2023-05-02 16:34 ` [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast " Lorenzo Stoakes
2023-05-02 17:13 ` David Hildenbrand
2023-05-02 17:22 ` Peter Zijlstra
2023-05-02 17:34 ` David Hildenbrand
2023-05-02 18:17 ` Lorenzo Stoakes
2023-05-02 19:07 ` David Hildenbrand
2023-05-02 19:07 ` Jason Gunthorpe
2023-05-02 19:25 ` Lorenzo Stoakes
2023-05-02 19:33 ` David Hildenbrand
2023-05-02 19:37 ` Lorenzo Stoakes
2023-05-02 23:51 ` Jason Gunthorpe
2023-05-03 0:22 ` Jason Gunthorpe
2023-05-02 18:59 ` Peter Zijlstra
2023-05-02 19:05 ` David Hildenbrand [this message]
2023-05-02 17:34 ` Lorenzo Stoakes
2023-05-02 17:31 ` Lorenzo Stoakes
2023-05-02 17:38 ` David Hildenbrand
2023-05-02 17:45 ` Lorenzo Stoakes
2023-05-02 19:17 ` David Hildenbrand
2023-05-02 19:45 ` Lorenzo Stoakes
2023-05-02 18:45 ` [PATCH v7 0/3] mm/gup: disallow GUP writing to file-backed mappings by default Matthew Rosato
2023-05-02 18:53 ` Lorenzo Stoakes
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