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>,
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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 20:17, 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

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