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

[David Hildenbrand]

On 18.03.22 21:29, Yang Shi wrote:
> On Thu, Mar 17, 2022 at 2:06 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 16.03.22 22:23, Yang Shi wrote:
>>> On Tue, Mar 15, 2022 at 3:52 AM David Hildenbrand <david@xxxxxxxxxx> 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.

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.

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

-- 
Thanks,

David / dhildenb