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

Vlastimil Babka
2022-04-20T17:10:35Z

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)