[PATCH 12/13] mm/thp: collapse_file() do try_to_unmap(TTU_BATCH_FLUSH)

Hugh Dickins
2022-02-06T21:51:53Z

collapse_file() is using unmap_mapping_pages(1) on each small page found
mapped, unlike others (reclaim, migration, splitting, memory-failure) who
use try_to_unmap(). There are four advantages to try_to_unmap(): first,
its TTU_IGNORE_MLOCK option now avoids leaving mlocked page in pagevec;
second, its vma lookup uses i_mmap_lock_read() not i_mmap_lock_write();
third, it breaks out early if page is not mapped everywhere it might be;
fourth, its TTU_BATCH_FLUSH option can be used, as in page reclaim, to
save up all the TLB flushing until all of the pages have been unmapped.

Wild guess: perhaps it was originally written to use try_to_unmap(),
but hit the VM_BUG_ON_PAGE(page_mapped) after unmapping, because without
TTU_SYNC it may skip page table locks; but unmap_mapping_pages() never
skips them, so fixed the issue. I did once hit that VM_BUG_ON_PAGE()
since making this change: we could pass TTU_SYNC here, but I think just
delete the check - the race is very rare, this is an ordinary small page
so we don't need to be so paranoid about mapcount surprises, and the
page_ref_freeze() just below already handles the case adequately.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/khugepaged.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d5e387c58bde..e0883a33efd6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1829,13 +1829,12 @@ static void collapse_file(struct mm_struct *mm,
}

if (page_mapped(page))
- unmap_mapping_pages(mapping, index, 1, false);
+ try_to_unmap(page, TTU_IGNORE_MLOCK | TTU_BATCH_FLUSH);

xas_lock_irq(&xas);
xas_set(&xas, index);

VM_BUG_ON_PAGE(page != xas_load(&xas), page);
- VM_BUG_ON_PAGE(page_mapped(page), page);

/*
* The page is expected to have page_count() == 3:
@@ -1899,6 +1898,13 @@ static void collapse_file(struct mm_struct *mm,
xas_unlock_irq(&xas);
xa_unlocked:

+ /*
+ * If collapse is successful, flush must be done now before copying.
+ * If collapse is unsuccessful, does flush actually need to be done?
+ * Do it anyway, to clear the state.
+ */
+ try_to_unmap_flush();
+
if (result == SCAN_SUCCEED) {
struct page *page, *tmp;

--
2.34.1