Re: [PATCH v2 11/15] mm: remember exclusively mapped anonymous pages with PG_anon_exclusive

[Yang Shi <shy828301@gmail.com>]

On Sat, Mar 19, 2022 at 3:21 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 18.03.22 21:29, Yang Shi wrote:
> > On Thu, Mar 17, 2022 at 2:06 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 16.03.22 22:23, Yang Shi wrote:
> >>> On Tue, Mar 15, 2022 at 3:52 AM David Hildenbrand <david@redhat.com> wrote:
> >>>>
> >>>> Let's mark exclusively mapped anonymous pages with PG_anon_exclusive as
> >>>> exclusive, and use that information to make GUP pins reliable and stay
> >>>> consistent with the page mapped into the page table even if the
> >>>> page table entry gets write-protected.
> >>>>
> >>>> With that information at hand, we can extend our COW logic to always
> >>>> reuse anonymous pages that are exclusive. For anonymous pages that
> >>>> might be shared, the existing logic applies.
> >>>>
> >>>> As already documented, PG_anon_exclusive is usually only expressive in
> >>>> combination with a page table entry. Especially PTE vs. PMD-mapped
> >>>> anonymous pages require more thought, some examples: due to mremap() we
> >>>> can easily have a single compound page PTE-mapped into multiple page tables
> >>>> exclusively in a single process -- multiple page table locks apply.
> >>>> Further, due to MADV_WIPEONFORK we might not necessarily write-protect
> >>>> all PTEs, and only some subpages might be pinned. Long story short: once
> >>>> PTE-mapped, we have to track information about exclusivity per sub-page,
> >>>> but until then, we can just track it for the compound page in the head
> >>>> page and not having to update a whole bunch of subpages all of the time
> >>>> for a simple PMD mapping of a THP.
> >>>>
> >>>> For simplicity, this commit mostly talks about "anonymous pages", while
> >>>> it's for THP actually "the part of an anonymous folio referenced via
> >>>> a page table entry".
> >>>>
> >>>> To not spill PG_anon_exclusive code all over the mm code-base, we let
> >>>> the anon rmap code to handle all PG_anon_exclusive logic it can easily
> >>>> handle.
> >>>>
> >>>> If a writable, present page table entry points at an anonymous (sub)page,
> >>>> that (sub)page must be PG_anon_exclusive. If GUP wants to take a reliably
> >>>> pin (FOLL_PIN) on an anonymous page references via a present
> >>>> page table entry, it must only pin if PG_anon_exclusive is set for the
> >>>> mapped (sub)page.
> >>>>
> >>>> This commit doesn't adjust GUP, so this is only implicitly handled for
> >>>> FOLL_WRITE, follow-up commits will teach GUP to also respect it for
> >>>> FOLL_PIN without !FOLL_WRITE, to make all GUP pins of anonymous pages
> >>>> fully reliable.
> >>>>
> >>>> Whenever an anonymous page is to be shared (fork(), KSM), or when
> >>>> temporarily unmapping an anonymous page (swap, migration), the relevant
> >>>> PG_anon_exclusive bit has to be cleared to mark the anonymous page
> >>>> possibly shared. Clearing will fail if there are GUP pins on the page:
> >>>> * For fork(), this means having to copy the page and not being able to
> >>>>   share it. fork() protects against concurrent GUP using the PT lock and
> >>>>   the src_mm->write_protect_seq.
> >>>> * For KSM, this means sharing will fail. For swap this means, unmapping
> >>>>   will fail, For migration this means, migration will fail early. All
> >>>>   three cases protect against concurrent GUP using the PT lock and a
> >>>>   proper clear/invalidate+flush of the relevant page table entry.
> >>>>
> >>>> This fixes memory corruptions reported for FOLL_PIN | FOLL_WRITE, when a
> >>>> pinned page gets mapped R/O and the successive write fault ends up
> >>>> replacing the page instead of reusing it. It improves the situation for
> >>>> O_DIRECT/vmsplice/... that still use FOLL_GET instead of FOLL_PIN,
> >>>> if fork() is *not* involved, however swapout and fork() are still
> >>>> problematic. Properly using FOLL_PIN instead of FOLL_GET for these
> >>>> GUP users will fix the issue for them.
> >>>>
> >>>> I. Details about basic handling
> >>>>
> >>>> I.1. Fresh anonymous pages
> >>>>
> >>>> page_add_new_anon_rmap() and hugepage_add_new_anon_rmap() will mark the
> >>>> given page exclusive via __page_set_anon_rmap(exclusive=1). As that is
> >>>> the mechanism fresh anonymous pages come into life (besides migration
> >>>> code where we copy the page->mapping), all fresh anonymous pages will
> >>>> start out as exclusive.
> >>>>
> >>>> I.2. COW reuse handling of anonymous pages
> >>>>
> >>>> When a COW handler stumbles over a (sub)page that's marked exclusive, it
> >>>> simply reuses it. Otherwise, the handler tries harder under page lock to
> >>>> detect if the (sub)page is exclusive and can be reused. If exclusive,
> >>>> page_move_anon_rmap() will mark the given (sub)page exclusive.
> >>>>
> >>>> Note that hugetlb code does not yet check for PageAnonExclusive(), as it
> >>>> still uses the old COW logic that is prone to the COW security issue
> >>>> because hugetlb code cannot really tolerate unnecessary/wrong COW as
> >>>> huge pages are a scarce resource.
> >>>>
> >>>> I.3. Migration handling
> >>>>
> >>>> try_to_migrate() has to try marking an exclusive anonymous page shared
> >>>> via page_try_share_anon_rmap(). If it fails because there are GUP pins
> >>>> on the page, unmap fails. migrate_vma_collect_pmd() and
> >>>> __split_huge_pmd_locked() are handled similarly.
> >>>>
> >>>> Writable migration entries implicitly point at shared anonymous pages.
> >>>> For readable migration entries that information is stored via a new
> >>>> "readable-exclusive" migration entry, specific to anonymous pages.
> >>>>
> >>>> When restoring a migration entry in remove_migration_pte(), information
> >>>> about exlusivity is detected via the migration entry type, and
> >>>> RMAP_EXCLUSIVE is set accordingly for
> >>>> page_add_anon_rmap()/hugepage_add_anon_rmap() to restore that
> >>>> information.
> >>>>
> >>>> I.4. Swapout handling
> >>>>
> >>>> try_to_unmap() has to try marking the mapped page possibly shared via
> >>>> page_try_share_anon_rmap(). If it fails because there are GUP pins on the
> >>>> page, unmap fails. For now, information about exclusivity is lost. In the
> >>>> future, we might want to remember that information in the swap entry in
> >>>> some cases, however, it requires more thought, care, and a way to store
> >>>> that information in swap entries.
> >>>>
> >>>> I.5. Swapin handling
> >>>>
> >>>> do_swap_page() will never stumble over exclusive anonymous pages in the
> >>>> swap cache, as try_to_migrate() prohibits that. do_swap_page() always has
> >>>> to detect manually if an anonymous page is exclusive and has to set
> >>>> RMAP_EXCLUSIVE for page_add_anon_rmap() accordingly.
> >>>>
> >>>> I.6. THP handling
> >>>>
> >>>> __split_huge_pmd_locked() has to move the information about exclusivity
> >>>> from the PMD to the PTEs.
> >>>>
> >>>> a) In case we have a readable-exclusive PMD migration entry, simply insert
> >>>> readable-exclusive PTE migration entries.
> >>>>
> >>>> b) In case we have a present PMD entry and we don't want to freeze
> >>>> ("convert to migration entries"), simply forward PG_anon_exclusive to
> >>>> all sub-pages, no need to temporarily clear the bit.
> >>>>
> >>>> c) In case we have a present PMD entry and want to freeze, handle it
> >>>> similar to try_to_migrate(): try marking the page shared first. In case
> >>>> we fail, we ignore the "freeze" instruction and simply split ordinarily.
> >>>> try_to_migrate() will properly fail because the THP is still mapped via
> >>>> PTEs.
> >>
> >> Hi,
> >>
> >> thanks for the review!
> >>
> >>>
> >>> How come will try_to_migrate() fail? The afterward pvmw will find
> >>> those PTEs then convert them to migration entries anyway IIUC.
> >>>
> >>
> >> It will run into that code:
> >>
> >>>> @@ -1903,6 +1938,15 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
> >>>>                                 page_vma_mapped_walk_done(&pvmw);
> >>>>                                 break;
> >>>>                         }
> >>>> +                       VM_BUG_ON_PAGE(pte_write(pteval) && PageAnon(page) &&
> >>>> +                                      !anon_exclusive, page);
> >>>> +                       if (anon_exclusive &&
> >>>> +                           page_try_share_anon_rmap(subpage)) {
> >>>> +                               set_pte_at(mm, address, pvmw.pte, pteval);
> >>>> +                               ret = false;
> >>>> +                               page_vma_mapped_walk_done(&pvmw);
> >>>> +                               break;
> >>>> +                       }
> >>
> >> and similarly fail the page_try_share_anon_rmap(), at which point
> >> try_to_migrate() stops and the caller will still observe a
> >> "page_mapped() == true".
> >
> > Thanks, I missed that. Yes, the page will still be mapped. This should
> > trigger the VM_WARN_ON_ONCE in unmap_page(), if this change will make
> > this happen more often, we may consider removing that warning even
> > though it is "once" since seeing a mapped page may become a normal
> > case (once DIO is switched to FOLL_PIN, it may be more often). Anyway
> > we don't have to remove it right now.
>
> Oh, very good catch! I wasn't able to trigger that warning in my testing
> so far. Interestingly, arch_unmap_one() could theoretically make this
> fail already and trigger the warning.

It should be very rare to trigger that warning. I just saw one report
before. And arch_unmap_one() actually can't fail except for Sparc. So
that warning was intended to catch some unusual issues, but this COW
patchset may make it much more usual than before.

>
> Apart from that warning, split_huge_page_to_list() should work as
> expected: freezing the refcount will fail if still mapped and we'll remap.

Yes, that was why it was changed to a WARN from a BUG, please see
commit 504e070dc08f ("mm: thp: replace DEBUG_VM BUG with VM_WARN when
unmap fails for split")

>
> I'll include a separate patch to just remove that VM_WARN_ON_ONCE -- thanks!

Thanks, I'm fine to remove it.

>
> --
> Thanks,
>
> David / dhildenb
>