[RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio

[Barry Song]

From: Barry Song <v-songbaohua@xxxxxxxx>

page_vma_mapped_walk() within try_to_unmap_one() races with other
PTEs modification such as break-before-make, while iterating PTEs
of a large folio, it will only begin to acquire PTL after it gets
a valid(present) PTE. break-before-make intermediately sets PTEs
to pte_none. Thus, a large folio's PTEs might be partially skipped
in try_to_unmap_one().
For example, for an anon folio, after try_to_unmap_one(), we may
have PTE0 present, while PTE1 ~ PTE(nr_pages - 1) are swap entries.
So folio will be still mapped, the folio fails to be reclaimed.
What’s even more worrying is, its PTEs are no longer in a unified
state. This might lead to accident folio_split() afterwards. And
since a part of PTEs are now swap entries, accessing them will
incur page fault - do_swap_page.
It creates both anxiety and more expense. While we can't avoid
userspace's unmap to break up unified PTEs such as CONT-PTE for
a large folio, we can indeed keep away from kernel's breaking up
them due to its code design.
This patch is holding PTL from PTE0, thus, the folio will either
be entirely reclaimed or entirely kept. On the other hand, this
approach doesn't increase PTL contention. Even w/o the patch,
page_vma_mapped_walk() will always get PTL after it sometimes
skips one or two PTEs because intermediate break-before-makes
are short, according to test. Of course, even w/o this patch,
the vast majority of try_to_unmap_one still can get PTL from
PTE0. This patch makes the number 100%.
The other option is that we can give up in try_to_unmap_one
once we find PTE0 is not the first entry we get PTL, we call
page_vma_mapped_walk_done() to end the iteration at this case.
This will keep the unified PTEs while the folio isn't reclaimed.
The result is quite similar with small folios with one PTE -
either entirely reclaimed or entirely kept.
Reclaiming large folios by holding PTL from PTE0 seems a better
option comparing to giving up after detecting PTL begins from
non-PTE0.

Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
 mm/vmscan.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0b888a2afa58..e4722fbbcd0c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1270,6 +1270,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 
 			if (folio_test_pmd_mappable(folio))
 				flags |= TTU_SPLIT_HUGE_PMD;
+			/*
+			 * if page table lock is not held from the first PTE of
+			 * a large folio, some PTEs might be skipped because of
+			 * races with break-before-make, for example, PTEs can
+			 * be pte_none intermediately, thus one or more PTEs
+			 * might be skipped in try_to_unmap_one, we might result
+			 * in a large folio is partially mapped and partially
+			 * unmapped after try_to_unmap
+			 */
+			if (folio_test_large(folio))
+				flags |= TTU_SYNC;
 
 			try_to_unmap(folio, flags);
 			if (folio_mapped(folio)) {
-- 
2.34.1