[PATCH 02/13] mm/munlock: delete FOLL_MLOCK and FOLL_POPULATE

Hugh Dickins
2022-02-06T21:32:24Z

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>
---
include/linux/mm.h | 2 --
mm/gup.c | 43 ++++++++-----------------------------------
mm/huge_memory.c | 33 ---------------------------------
3 files changed, 8 insertions(+), 70 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 213cc569b192..74ee50c2033b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2925,13 +2925,11 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO
* and return without waiting upon it */
-#define FOLL_POPULATE 0x40 /* fault in pages (with FOLL_MLOCK) */
#define FOLL_NOFAULT 0x80 /* do not fault in pages */
#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */
#define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
-#define FOLL_MLOCK 0x1000 /* lock present pages */
#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
#define FOLL_COW 0x4000 /* internal GUP flag */
#define FOLL_ANON 0x8000 /* don't do file mappings */
diff --git a/mm/gup.c b/mm/gup.c
index f0af462ac1e2..2076902344d8 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -572,32 +572,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
*/
mark_page_accessed(page);
}
- if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
- /* Do not mlock pte-mapped THP */
- if (PageTransCompound(page))
- goto out;
-
- /*
- * The preliminary mapping check is mainly to avoid the
- * pointless overhead of lock_page on the ZERO_PAGE
- * which might bounce very badly if there is contention.
- *
- * If the page is already locked, we don't need to
- * handle it now - vmscan will handle it later if and
- * when it attempts to reclaim the page.
- */
- if (page->mapping && trylock_page(page)) {
- lru_add_drain(); /* push cached pages to LRU */
- /*
- * Because we lock page here, and migration is
- * blocked by the pte's page reference, and we
- * know the page is still mapped, we don't even
- * need to check for file-cache page truncation.
- */
- mlock_vma_page(page);
- unlock_page(page);
- }
- }
out:
pte_unmap_unlock(ptep, ptl);
return page;
@@ -920,9 +894,6 @@ static int faultin_page(struct vm_area_struct *vma,
unsigned int fault_flags = 0;
vm_fault_t ret;

- /* mlock all present pages, but do not fault in new pages */
- if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
- return -ENOENT;
if (*flags & FOLL_NOFAULT)
return -EFAULT;
if (*flags & FOLL_WRITE)
@@ -1173,8 +1144,6 @@ static long __get_user_pages(struct mm_struct *mm,
case -ENOMEM:
case -EHWPOISON:
goto out;
- case -ENOENT:
- goto next_page;
}
BUG();
} else if (PTR_ERR(page) == -EEXIST) {
@@ -1472,9 +1441,14 @@ long populate_vma_page_range(struct vm_area_struct *vma,
VM_BUG_ON_VMA(end > vma->vm_end, vma);
mmap_assert_locked(mm);

- gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
+ /*
+ * Rightly or wrongly, the VM_LOCKONFAULT case has never used
+ * faultin_page() to break COW, so it has no work to do here.
+ */
if (vma->vm_flags & VM_LOCKONFAULT)
- gup_flags &= ~FOLL_POPULATE;
+ return nr_pages;
+
+ gup_flags = FOLL_TOUCH;
/*
* We want to touch writable mappings with a write fault in order
* to break COW, except for shared mappings because these don't COW
@@ -1541,10 +1515,9 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
* in the page table.
* FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit
* a poisoned page.
- * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT.
* !FOLL_FORCE: Require proper access permissions.
*/
- gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON;
+ gup_flags = FOLL_TOUCH | FOLL_HWPOISON;
if (write)
gup_flags |= FOLL_WRITE;

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);

--
2.34.1