Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

From: David Hildenbrand
Date: Thu Jun 06 2024 - 04:03:08 EST

On 06.06.24 05:55, Lance Yang wrote:
On Wed, Jun 5, 2024 at 10:28 PM David Hildenbrand <david@xxxxxxxxxx> wrote:

On 05.06.24 16:20, Lance Yang wrote:
Hi David,

On Wed, Jun 5, 2024 at 8:46 PM David Hildenbrand <david@xxxxxxxxxx> wrote:

On 21.05.24 06:02, Lance Yang wrote:
In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
folios, start the pagewalk first, then call split_huge_pmd_address() to
split the folio.

Since TTU_SPLIT_HUGE_PMD will no longer perform immediately, we might
encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range during
the page walk. It’s probably necessary to mlock this THP to prevent it from
being picked up during page reclaim.

Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Suggested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
---

[...] again, sorry for the late review.

No worries at all, thanks for taking time to review!


diff --git a/mm/rmap.c b/mm/rmap.c
index ddffa30c79fb..08a93347f283 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
if (flags & TTU_SYNC)
pvmw.flags = PVMW_SYNC;

- if (flags & TTU_SPLIT_HUGE_PMD)
- split_huge_pmd_address(vma, address, false, folio);
-
/*
* For THP, we have to assume the worse case ie pmd for invalidation.
* For hugetlb, it could be much worse if we need to do pud
@@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
mmu_notifier_invalidate_range_start(&range);

while (page_vma_mapped_walk(&pvmw)) {
- /* Unexpected PMD-mapped THP? */
- VM_BUG_ON_FOLIO(!pvmw.pte, folio);
-
/*
* If the folio is in an mlock()d vma, we must not swap it out.
*/
if (!(flags & TTU_IGNORE_MLOCK) &&
(vma->vm_flags & VM_LOCKED)) {
/* Restore the mlock which got missed */
- if (!folio_test_large(folio))
+ if (!folio_test_large(folio) ||
+ (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
mlock_vma_folio(folio, vma);

Can you elaborate why you think this would be required? If we would have
performed the split_huge_pmd_address() beforehand, we would still be
left with a large folio, no?

Yep, there would still be a large folio, but it wouldn't be PMD-mapped.

After Weifeng's series[1], the kernel supports mlock for PTE-mapped large
folio, but there are a few scenarios where we don't mlock a large folio, such
as when it crosses a VM_LOCKed VMA boundary.

- if (!folio_test_large(folio))
+ if (!folio_test_large(folio) ||
+ (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))

And this check is just future-proofing and likely unnecessary. If encountering a
PMD-mapped THP missing the mlock for some reason, we can mlock this
THP to prevent it from being picked up during page reclaim, since it is fully
mapped and doesn't cross the VMA boundary, IIUC.

What do you think?
I would appreciate any suggestions regarding this check ;)

Reading this patch only, I wonder if this change makes sense in the
context here.

Allow me to try explaining it again ;)


Before this patch, we would have PTE-mapped the PMD-mapped THP before
reaching this call and skipped it due to "!folio_test_large(folio)".

Yes, there is only a PTE-mapped THP when doing the "!folio_test_large(folio)"
check, as we will first conditionally split the PMD via
split_huge_pmd_address().


After this patch, we either

Things will change. We'll first do the "!folio_test_large(folio)" check, then
conditionally split the PMD via split_huge_pmd_address().


a) PTE-remap the THP after this check, but retry and end-up here again,
whereby we would skip it due to "!folio_test_large(folio)".

Hmm...

IIUC, we will skip it after this check, stop the page walk, and not
PTE-remap the THP.


b) Discard the PMD-mapped THP due to lazyfree directly. Can that
co-exist with mlock and what would be the problem here with mlock?

Before discarding a PMD-mapped THP as a whole, as patch #3 did,
we also perform the "!folio_test_large(folio)" check. If the THP coexists
with mlock, we will skip it, stop the page walk, and not discard it. IIUC.

But "!folio_test_large(folio)" would *skip* the THP and not consider it regarding mlock.

I'm probably missing something and should try current mm/mm-unstable with MADV_FREE + mlock() on a PMD-mapped THP.




So if the check is required in this patch, we really have to understand
why. If not, we should better drop it from this patch.

I added the "!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD))" check
in this patch just to future-proof mlock for a PMD-mapped THP missing
the mlock, to prevent it from being picked up during page reclaim.

But is this really required? It seems like nothing should really be broken
without this check.

Perhaps, we should drop it from this patch until we fully understand the
reason for it. Could you get me some suggestions?

We should drop it from this patch, agreed. We might need it ("!pvmw.pte") in patch #3, but I still have to understand if there really would be a problem.

--
Cheers,

David / dhildenb