mm/munlock: delete FOLL_MLOCK and FOLL_POPULATE

If counting page mlocks, we must not double-count: follow_page_pte() can
tell if a page has already been Mlocked or not, but cannot tell if a pte
has already been counted or not: that will have to be done when the pte
is mapped in (which lru_cache_add_inactive_or_unevictable() already tracks
for new anon pages, but there's no such tracking yet for others).

Delete all the FOLL_MLOCK code - faulting in the missing pages will do
all that is necessary, without special mlock_vma_page() calls from here.

But then FOLL_POPULATE turns out to serve no purpose - it was there so
that its absence would tell faultin_page() not to faultin page when
setting up VM_LOCKONFAULT areas; but if there's no special work needed
here for mlock, then there's no work at all here for VM_LOCKONFAULT.

Have I got that right?  I've not looked into the history, but see that
FOLL_POPULATE goes back before VM_LOCKONFAULT: did it serve a different
purpose before?  Ah, yes, it was used to skip the old stack guard page.

And is it intentional that COW is not broken on existing pages when
setting up a VM_LOCKONFAULT area?  I can see that being argued either
way, and have no reason to disagree with current behaviour.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

1 files changed, 0 insertions, 33 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 406a3c28c026..9a34b85ebcf8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1380,39 +1380,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 	if (flags & FOLL_TOUCH)
 		touch_pmd(vma, addr, pmd, flags);
 
-	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
-		/*
-		 * We don't mlock() pte-mapped THPs. This way we can avoid
-		 * leaking mlocked pages into non-VM_LOCKED VMAs.
-		 *
-		 * For anon THP:
-		 *
-		 * In most cases the pmd is the only mapping of the page as we
-		 * break COW for the mlock() -- see gup_flags |= FOLL_WRITE for
-		 * writable private mappings in populate_vma_page_range().
-		 *
-		 * The only scenario when we have the page shared here is if we
-		 * mlocking read-only mapping shared over fork(). We skip
-		 * mlocking such pages.
-		 *
-		 * For file THP:
-		 *
-		 * We can expect PageDoubleMap() to be stable under page lock:
-		 * for file pages we set it in page_add_file_rmap(), which
-		 * requires page to be locked.
-		 */
-
-		if (PageAnon(page) && compound_mapcount(page) != 1)
-			goto skip_mlock;
-		if (PageDoubleMap(page) || !page->mapping)
-			goto skip_mlock;
-		if (!trylock_page(page))
-			goto skip_mlock;
-		if (page->mapping && !PageDoubleMap(page))
-			mlock_vma_page(page);
-		unlock_page(page);
-	}
-skip_mlock:
 	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
 	VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);