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mm/munlock: delete FOLL_MLOCK and FOLL_POPULATE

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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>
This commit is contained in:
Hugh Dickins 2022-02-14 18:21:52 -08:00 committed by Matthew Wilcox (Oracle)
parent ebcbc6ea7d
commit b67bf49ce7
3 changed files with 8 additions and 70 deletions
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2
include/linux/mm.h
View File

@ -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 */

43
mm/gup.c
View File

@ -597,32 +597,6 @@ retry:
*/
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;
@ -945,9 +919,6 @@ 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)
@ -1198,8 +1169,6 @@ retry:
case -ENOMEM:
case -EHWPOISON:
goto out;
case -ENOENT:
goto next_page;
}
BUG();
} else if (PTR_ERR(page) == -EEXIST) {
@ -1497,9 +1466,14 @@ 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
@ -1566,10 +1540,9 @@ 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;

33
mm/huge_memory.c
View File

@ -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);