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Subject[PATCH v5 03/11] mm: thp: introduce separate TTU flag for thp freezing
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From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

TTU_MIGRATION is used to convert pte into migration entry until thp split
completes. This behavior conflicts with thp migration added later patches,
so let's introduce a new TTU flag specifically for freezing.

try_to_unmap() is used both for thp split (via freeze_page()) and page
migration (via __unmap_and_move()). In freeze_page(), ttu_flag given for
head page is like below (assuming anonymous thp):

(TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
TTU_MIGRATION | TTU_SPLIT_HUGE_PMD)

and ttu_flag given for tail pages is:

(TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
TTU_MIGRATION)

__unmap_and_move() calls try_to_unmap() with ttu_flag:

(TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS)

Now I'm trying to insert a branch for thp migration at the top of
try_to_unmap_one() like below

static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
unsigned long address, void *arg)
{
...
if (flags & TTU_MIGRATION) {
if (!pvmw.pte && page) {
set_pmd_migration_entry(&pvmw, page);
continue;
}
}

, so try_to_unmap() for tail pages called by thp split can go into thp
migration code path (which converts *pmd* into migration entry), while
the expectation is to freeze thp (which converts *pte* into migration entry.)

I detected this failure as a "bad page state" error in a testcase where
split_huge_page() is called from queue_pages_pte_range().

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
include/linux/rmap.h | 3 ++-
mm/huge_memory.c | 2 +-
mm/rmap.c | 7 ++++---
3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 43ef2c30cb0f..f8ca2e74b819 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -93,8 +93,9 @@ enum ttu_flags {
TTU_BATCH_FLUSH = 0x40, /* Batch TLB flushes where possible
* and caller guarantees they will
* do a final flush if necessary */
- TTU_RMAP_LOCKED = 0x80 /* do not grab rmap lock:
+ TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock:
* caller holds it */
+ TTU_SPLIT_FREEZE = 0x100, /* freeze pte under splitting thp */
};

#ifdef CONFIG_MMU
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1c19331a2db9..0db1f1c90aad 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2159,7 +2159,7 @@ static void freeze_page(struct page *page)
VM_BUG_ON_PAGE(!PageHead(page), page);

if (PageAnon(page))
- ttu_flags |= TTU_MIGRATION;
+ ttu_flags |= TTU_SPLIT_FREEZE;

unmap_success = try_to_unmap(page, ttu_flags);
VM_BUG_ON_PAGE(!unmap_success, page);
diff --git a/mm/rmap.c b/mm/rmap.c
index 5c97ce4f5b2d..b0c6b20dca74 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1308,7 +1308,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,

if (flags & TTU_SPLIT_HUGE_PMD) {
split_huge_pmd_address(vma, address,
- flags & TTU_MIGRATION, page);
+ flags & TTU_SPLIT_FREEZE, page);
}

while (page_vma_mapped_walk(&pvmw)) {
@@ -1394,7 +1394,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
*/
dec_mm_counter(mm, mm_counter(page));
} else if (IS_ENABLED(CONFIG_MIGRATION) &&
- (flags & TTU_MIGRATION)) {
+ (flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))) {
swp_entry_t entry;
pte_t swp_pte;
/*
@@ -1519,7 +1519,8 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
* locking requirements of exec(), migration skips
* temporary VMAs until after exec() completes.
*/
- if ((flags & TTU_MIGRATION) && !PageKsm(page) && PageAnon(page))
+ if ((flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))
+ && !PageKsm(page) && PageAnon(page))
rwc.invalid_vma = invalid_migration_vma;

if (flags & TTU_RMAP_LOCKED)
--
2.11.0
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