[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <4c032450-7184-fdb9-7b50-670ff06fc225@redhat.com>
Date: Tue, 2 May 2023 21:07:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lstoakes@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, 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>,
Matthew Wilcox <willy@...radead.org>,
Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
Leon Romanovsky <leon@...nel.org>,
Christian Benvenuti <benve@...co.com>,
Nelson Escobar <neescoba@...co.com>,
Bernard Metzler <bmt@...ich.ibm.com>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Bjorn Topel <bjorn@...nel.org>,
Magnus Karlsson <magnus.karlsson@...el.com>,
Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
Jonathan Lemon <jonathan.lemon@...il.com>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Christian Brauner <brauner@...nel.org>,
Richard Cochran <richardcochran@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
netdev@...r.kernel.org, bpf@...r.kernel.org,
Oleg Nesterov <oleg@...hat.com>,
Jason Gunthorpe <jgg@...dia.com>,
John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Pavel Begunkov <asml.silence@...il.com>,
Mika Penttila <mpenttil@...hat.com>,
Dave Chinner <david@...morbit.com>,
Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>,
Matthew Rosato <mjrosato@...ux.ibm.com>,
"Paul E . McKenney" <paulmck@...nel.org>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
file-backed mappings
On 02.05.23 20:17, Lorenzo Stoakes wrote:
> On Tue, May 02, 2023 at 07:34:06PM +0200, David Hildenbrand wrote:
>> On 02.05.23 19:22, Peter Zijlstra wrote:
>>> On Tue, May 02, 2023 at 07:13:49PM +0200, David Hildenbrand wrote:
>>>> [...]
>>>>
>>>>> +{
>>>>> + struct address_space *mapping;
>>>>> +
>>>>> + /*
>>>>> + * GUP-fast disables IRQs - this prevents IPIs from causing page tables
>>>>> + * to disappear from under us, as well as preventing RCU grace periods
>>>>> + * from making progress (i.e. implying rcu_read_lock()).
>>>>> + *
>>>>> + * This means we can rely on the folio remaining stable for all
>>>>> + * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE
>>>>> + * and those that do not.
>>>>> + *
>>>>> + * We get the added benefit that given inodes, and thus address_space,
>>>>> + * objects are RCU freed, we can rely on the mapping remaining stable
>>>>> + * here with no risk of a truncation or similar race.
>>>>> + */
>>>>> + lockdep_assert_irqs_disabled();
>>>>> +
>>>>> + /*
>>>>> + * If no mapping can be found, this implies an anonymous or otherwise
>>>>> + * non-file backed folio so in this instance we permit the pin.
>>>>> + *
>>>>> + * shmem and hugetlb mappings do not require dirty-tracking so we
>>>>> + * explicitly whitelist these.
>>>>> + *
>>>>> + * Other non dirty-tracked folios will be picked up on the slow path.
>>>>> + */
>>>>> + mapping = folio_mapping(folio);
>>>>> + return !mapping || shmem_mapping(mapping) || folio_test_hugetlb(folio);
>>>>
>>>> "Folios in the swap cache return the swap mapping" -- you might disallow
>>>> pinning anonymous pages that are in the swap cache.
>>>>
>>>> I recall that there are corner cases where we can end up with an anon page
>>>> that's mapped writable but still in the swap cache ... so you'd fallback to
>>>> the GUP slow path (acceptable for these corner cases, I guess), however
>>>> especially the comment is a bit misleading then.
>>>>
>>>> So I'd suggest not dropping the folio_test_anon() check, or open-coding it
>>>> ... which will make this piece of code most certainly easier to get when
>>>> staring at folio_mapping(). Or to spell it out in the comment (usually I
>>>> prefer code over comments).
>>>
>>> So how stable is folio->mapping at this point? Can two subsequent reads
>>> get different values? (eg. an actual mapping and NULL)
>>>
>>> If so, folio_mapping() itself seems to be missing a READ_ONCE() to avoid
>>> the compiler from emitting the load multiple times.
>>
>> I can only talk about anon pages in this specific call order here (check
>> first, then test if the PTE changed in the meantime): we don't care if we
>> get two different values. If we get a different value the second time,
>> surely we (temporarily) pinned an anon page that is no longer mapped (freed
>> in the meantime). But in that case (even if we read garbage folio->mapping
>> and made the wrong call here), we'll detect afterwards that the PTE changed,
>> and unpin what we (temporarily) pinned. As folio_test_anon() only checks two
>> bits in folio->mapping it's fine, because we won't dereference garbage
>> folio->mapping.
>>
>> With folio_mapping() on !anon and READ_ONCE() ... good question. Kirill said
>> it would be fairly stable, but I suspect that it could change (especially if
>> we call it before validating if the PTE changed as I described further
>> below).
>>
>> 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").
>>
>
> On a specific point - if mapping turns out to be NULL after we confirm
> stable PTE, I'd be inclined to reject and let the slow path take care of
> it, would you agree that that's the correct approach?
If it's not an anon page and the mapping is NULL, I'd say simply
fallback to the slow path.
--
Thanks,
David / dhildenb
Powered by blists - more mailing lists