Re: [PATCH v2 1/8] mm/swap: remember PG_anon_exclusive via a swp pte bit

[David Hildenbrand <david@redhat.com>]

On 20.04.22 19:10, Vlastimil Babka wrote:
> On 3/29/22 18:43, David Hildenbrand wrote:
>> Currently, we clear PG_anon_exclusive in try_to_unmap() and forget about
>> it. We do this, to keep fork() logic on swap entries easy and efficient:
>> for example, if we wouldn't clear it when unmapping, we'd have to lookup
>> the page in the swapcache for each and every swap entry during fork() and
>> clear PG_anon_exclusive if set.
>>
>> Instead, we want to store that information directly in the swap pte,
>> protected by the page table lock, similarly to how we handle
>> SWP_MIGRATION_READ_EXCLUSIVE for migration entries. However, for actual
>> swap entries, we don't want to mess with the swap type (e.g., still one
>> bit) because it overcomplicates swap code.
>>
>> In try_to_unmap(), we already reject to unmap in case the page might be
>> pinned, because we must not lose PG_anon_exclusive on pinned pages ever.
>> Checking if there are other unexpected references reliably *before*
>> completely unmapping a page is unfortunately not really possible: THP
>> heavily overcomplicate the situation. Once fully unmapped it's easier --
>> we, for example, make sure that there are no unexpected references
>> *after* unmapping a page before starting writeback on that page.
>>
>> So, we currently might end up unmapping a page and clearing
>> PG_anon_exclusive if that page has additional references, for example,
>> due to a FOLL_GET.
>>
>> do_swap_page() has to re-determine if a page is exclusive, which will
>> easily fail if there are other references on a page, most prominently
>> GUP references via FOLL_GET. This can currently result in memory
>> corruptions when taking a FOLL_GET | FOLL_WRITE reference on a page even
>> when fork() is never involved: try_to_unmap() will succeed, and when
>> refaulting the page, it cannot be marked exclusive and will get replaced
>> by a copy in the page tables on the next write access, resulting in writes
>> via the GUP reference to the page being lost.
>>
>> In an ideal world, everybody that uses GUP and wants to modify page
>> content, such as O_DIRECT, would properly use FOLL_PIN. However, that
>> conversion will take a while. It's easier to fix what used to work in the
>> past (FOLL_GET | FOLL_WRITE) remembering PG_anon_exclusive. In addition,
>> by remembering PG_anon_exclusive we can further reduce unnecessary COW
>> in some cases, so it's the natural thing to do.
>>
>> So let's transfer the PG_anon_exclusive information to the swap pte and
>> store it via an architecture-dependant pte bit; use that information when
>> restoring the swap pte in do_swap_page() and unuse_pte(). During fork(), we
>> simply have to clear the pte bit and are done.
>>
>> Of course, there is one corner case to handle: swap backends that don't
>> support concurrent page modifications while the page is under writeback.
>> Special case these, and drop the exclusive marker. Add a comment why that
>> is just fine (also, reuse_swap_page() would have done the same in the
>> past).
>>
>> In the future, we'll hopefully have all architectures support
>> __HAVE_ARCH_PTE_SWP_EXCLUSIVE, such that we can get rid of the empty
>> stubs and the define completely. Then, we can also convert
>> SWP_MIGRATION_READ_EXCLUSIVE. For architectures it's fairly easy to
>> support: either simply use a yet unused pte bit that can be used for swap
>> entries, steal one from the arch type bits if they exceed 5, or steal one
>> from the offset bits.
>>
>> Note: R/O FOLL_GET references were never really reliable, especially
>> when taking one on a shared page and then writing to the page (e.g., GUP
>> after fork()). FOLL_GET, including R/W references, were never really
>> reliable once fork was involved (e.g., GUP before fork(),
>> GUP during fork()). KSM steps back in case it stumbles over unexpected
>> references and is, therefore, fine.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> With the fixup as reportedy by Miaohe Lin
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
> (sent a separate mm-commits mail to inquire about the fix going missing from
> mmotm)
> 
> https://lore.kernel.org/mm-commits/c3195d8a-2931-0749-973a-1d04e4baec94@suse.cz/T/#m4e98ccae6f747e11f45e4d0726427ba2fef740eb

Yes I saw that, thanks for catching that!


-- 
Thanks,

David / dhildenb