Re: [PATCH RFC v2 5/9] mm/huge_memory: streamline COW logic in do_huge_pmd_wp_page()

[David Hildenbrand <david@redhat.com>]

On 26.01.22 21:36, Yang Shi wrote:
> On Wed, Jan 26, 2022 at 2:00 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> We currently have a different COW logic for anon THP than we have for
>> ordinary anon pages in do_wp_page(): the effect is that the issue reported
>> in CVE-2020-29374 is currently still possible for anon THP: an unintended
>> information leak from the parent to the child.
>>
>> Let's apply the same logic (page_count() == 1), with similar
>> optimizations to remove additional references first as we really want to
>> avoid PTE-mapping the THP and copying individual pages best we can.
>>
>> If we end up with a page that has page_count() != 1, we'll have to PTE-map
>> the THP and fallback to do_wp_page(), which will always copy the page.
>>
>> Note that KSM does not apply to THP.
>>
>> I. Interaction with the swapcache and writeback
>>
>> While a THP is in the swapcache, the swapcache holds one reference on each
>> subpage of the THP. So with PageSwapCache() set, we expect as many
>> additional references as we have subpages. If we manage to remove the
>> THP from the swapcache, all these references will be gone.
>>
>> Usually, a THP is not split when entered into the swapcache and stays a
>> compound page. However, try_to_unmap() will PTE-map the THP and use PTE
>> swap entries. There are no PMD swap entries for that purpose, consequently,
>> we always only swapin subpages into PTEs.
>>
>> Removing a page from the swapcache can fail either when there are remaining
>> swap entries (in which case COW is the right thing to do) or if the page is
>> currently under writeback.
>>
>> Having a locked, R/O PMD-mapped THP that is in the swapcache seems to be
>> possible only in corner cases, for example, if try_to_unmap() failed
>> after adding the page to the swapcache. However, it's comparatively easy to
>> handle.
>>
>> As we have to fully unmap a THP before starting writeback, and swapin is
>> always done on the PTE level, we shouldn't find a R/O PMD-mapped THP in the
>> swapcache that is under writeback. This should at least leave writeback
>> out of the picture.
>>
>> II. Interaction with GUP references
>>
>> Having a R/O PMD-mapped THP with GUP references (i.e., R/O references)
>> will result in PTE-mapping the THP on a write fault. Similar to ordinary
>> anon pages, do_wp_page() will have to copy sub-pages and result in a
>> disconnect between the GUP references and the pages actually mapped into
>> the page tables. To improve the situation in the future, we'll need
>> additional handling to mark anonymous pages as definitely exclusive to a
>> single process, only allow GUP pins on exclusive anon pages, and
>> disallow sharing of exclusive anon pages with GUP pins e.g., during
>> fork().
>>
>> III. Interaction with references from LRU pagevecs
>>
>> Similar to ordinary anon pages, we can have LRU pagevecs referencing our
>> THP. Reliably removing such references requires draining LRU pagevecs on
>> all CPUs -- lru_add_drain_all() -- a possibly expensive operation that can
>> sleep. For now, similar do do_wp_page(), let's conditionally drain the
>> local LRU pagevecs only if we detect !PageLRU().
>>
>> IV. Interaction with speculative/temporary references
>>
>> Similar to ordinary anon pages, other speculative/temporary references on
>> the THP, for example, from the pagecache or page migration code, will
>> disallow exclusive reuse of the page. We'll have to PTE-map the THP.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  mm/huge_memory.c | 19 +++++++++++++++----
>>  1 file changed, 15 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 406a3c28c026..b6ba88a98266 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1286,6 +1286,7 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>         struct page *page;
>>         unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>         pmd_t orig_pmd = vmf->orig_pmd;
>> +       int swapcache_refs = 0;
>>
>>         vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>>         VM_BUG_ON_VMA(!vma->anon_vma, vma);
>> @@ -1303,7 +1304,6 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>         page = pmd_page(orig_pmd);
>>         VM_BUG_ON_PAGE(!PageHead(page), page);
>>
>> -       /* Lock page for reuse_swap_page() */
>>         if (!trylock_page(page)) {
>>                 get_page(page);
>>                 spin_unlock(vmf->ptl);
>> @@ -1319,10 +1319,20 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>         }
>>
>>         /*
>> -        * We can only reuse the page if nobody else maps the huge page or it's
>> -        * part.
>> +        * See do_wp_page(): we can only map the page writable if there are
>> +        * no additional references.
>>          */
>> -       if (reuse_swap_page(page)) {
>> +       if (PageSwapCache(page))
>> +               swapcache_refs = thp_nr_pages(page);
>> +       if (page_count(page) > 1 + swapcache_refs + !PageLRU(page))
>> +               goto unlock_fallback;
>> +       if (!PageLRU(page))
>> +               lru_add_drain();
> 
> IMHO, draining lru doesn't help out too much for THP since THP will be
> drained to LRU immediately once it is added into pagevec.

Oh, thanks, I think you're right. The interesting bit is

static bool pagevec_add_and_need_flush(struct pagevec *pvec, struct page
*page)
{
	bool ret = false;

	if (!pagevec_add(pvec, page) || PageCompound(page) ||
			lru_cache_disabled())
		ret = true;

	return ret;
}

Which indeed drains after adding it to the pagevec.

Will adjust the patch and update the description/comment accordingly,
thanks!

-- 
Thanks,

David / dhildenb