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mm/thp: collapse_file() do try_to_unmap(TTU_BATCH_FLUSH) 

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>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
This commit is contained in:
Hugh Dickins 2022-02-14 18:40:55 -08:00 committed by Matthew Wilcox (Oracle)
parent b74355078b
commit 6d9df8a588
1 changed files with 8 additions and 2 deletions
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10
mm/khugepaged.c
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@ -1834,13 +1834,12 @@ 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:
@ -1904,6 +1903,13 @@ xa_locked:
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;