[PATCH 3/4] mm: clear_page_mlock in page_remove_rmap
From: Hugh Dickins <hughd@google.com>
Date: 2012-09-19 03:55:58
Also in:
lkml
We had thought that pages could no longer get freed while still marked
as mlocked; but Johannes Weiner posted this program to demonstrate that
truncating an mlocked private file mapping containing COWed pages is
still mishandled:
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
int main(void)
{
char *map;
int fd;
system("grep mlockfreed /proc/vmstat");
fd = open("chigurh", O_CREAT|O_EXCL|O_RDWR);
unlink("chigurh");
ftruncate(fd, 4096);
map = mmap(NULL, 4096, PROT_WRITE, MAP_PRIVATE, fd, 0);
map[0] = 11;
mlock(map, sizeof(fd));
ftruncate(fd, 0);
close(fd);
munlock(map, sizeof(fd));
munmap(map, 4096);
system("grep mlockfreed /proc/vmstat");
return 0;
}
The anon COWed pages are not caught by truncation's clear_page_mlock()
of the pagecache pages; but unmap_mapping_range() unmaps them, so we
ought to look out for them there in page_remove_rmap(). Indeed, why
should truncation or invalidation be doing the clear_page_mlock() when
removing from pagecache? mlock is a property of mapping in userspace,
not a propertly of pagecache: an mlocked unmapped page is nonsensical.
Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <redacted>
Cc: Rik van Riel <redacted>
Cc: Michel Lespinasse <redacted>
Cc: Ying Han <redacted>
---
mm/internal.h | 7 +------
mm/memory.c | 10 +++++-----
mm/mlock.c | 16 +++-------------
mm/rmap.c | 4 ++++
mm/truncate.c | 4 ----
5 files changed, 13 insertions(+), 28 deletions(-)
--- 3.6-rc6.orig/mm/internal.h 2012-09-18 16:39:50.000000000 -0700
+++ 3.6-rc6/mm/internal.h 2012-09-18 17:51:02.871288773 -0700@@ -200,12 +200,7 @@ extern void munlock_vma_page(struct page * If called for a page that is still mapped by mlocked vmas, all we do * is revert to lazy LRU behaviour -- semantics are not broken. */ -extern void __clear_page_mlock(struct page *page); -static inline void clear_page_mlock(struct page *page) -{ - if (unlikely(TestClearPageMlocked(page))) - __clear_page_mlock(page); -} +extern void clear_page_mlock(struct page *page); /* * mlock_migrate_page - called only from migrate_page_copy() to --- 3.6-rc6.orig/mm/memory.c 2012-09-18 15:38:08.000000000 -0700 +++ 3.6-rc6/mm/memory.c 2012-09-18 17:51:02.871288773 -0700
@@ -1576,12 +1576,12 @@ split_fallthrough: 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, we need - * only check for file-cache page truncation. + * 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. */ - if (page->mapping) - mlock_vma_page(page); + mlock_vma_page(page); unlock_page(page); } } --- 3.6-rc6.orig/mm/mlock.c 2012-09-18 15:38:08.000000000 -0700 +++ 3.6-rc6/mm/mlock.c 2012-09-18 17:51:02.871288773 -0700
@@ -51,13 +51,10 @@ EXPORT_SYMBOL(can_do_mlock); /* * LRU accounting for clear_page_mlock() */ -void __clear_page_mlock(struct page *page) +void clear_page_mlock(struct page *page) { - VM_BUG_ON(!PageLocked(page)); - - if (!page->mapping) { /* truncated ? */ + if (!TestClearPageMlocked(page)) return; - } dec_zone_page_state(page, NR_MLOCK); count_vm_event(UNEVICTABLE_PGCLEARED);
@@ -290,14 +287,7 @@ void munlock_vma_pages_range(struct vm_a page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP); if (page && !IS_ERR(page)) { lock_page(page); - /* - * Like in __mlock_vma_pages_range(), - * because we lock page here and migration is - * blocked by the elevated reference, we need - * only check for file-cache page truncation. - */ - if (page->mapping) - munlock_vma_page(page); + munlock_vma_page(page); unlock_page(page); put_page(page); } --- 3.6-rc6.orig/mm/rmap.c 2012-09-18 16:39:50.000000000 -0700 +++ 3.6-rc6/mm/rmap.c 2012-09-18 17:51:02.871288773 -0700
@@ -1203,7 +1203,10 @@ void page_remove_rmap(struct page *page) } else { __dec_zone_page_state(page, NR_FILE_MAPPED); mem_cgroup_dec_page_stat(page, MEMCG_NR_FILE_MAPPED); + mem_cgroup_end_update_page_stat(page, &locked, &flags); } + if (unlikely(PageMlocked(page))) + clear_page_mlock(page); /* * It would be tidy to reset the PageAnon mapping here, * but that might overwrite a racing page_add_anon_rmap
@@ -1213,6 +1216,7 @@ void page_remove_rmap(struct page *page) * Leaving it set also helps swapoff to reinstate ptes * faster for those pages still in swapcache. */ + return; out: if (!anon) mem_cgroup_end_update_page_stat(page, &locked, &flags); --- 3.6-rc6.orig/mm/truncate.c 2012-09-18 15:42:17.000000000 -0700 +++ 3.6-rc6/mm/truncate.c 2012-09-18 17:51:02.875288902 -0700
@@ -107,7 +107,6 @@ truncate_complete_page(struct address_sp cancel_dirty_page(page, PAGE_CACHE_SIZE); - clear_page_mlock(page); ClearPageMappedToDisk(page); delete_from_page_cache(page); return 0;
@@ -132,7 +131,6 @@ invalidate_complete_page(struct address_ if (page_has_private(page) && !try_to_release_page(page, 0)) return 0; - clear_page_mlock(page); ret = remove_mapping(mapping, page); return ret;
@@ -394,8 +392,6 @@ invalidate_complete_page2(struct address if (page_has_private(page) && !try_to_release_page(page, GFP_KERNEL)) return 0; - clear_page_mlock(page); - spin_lock_irq(&mapping->tree_lock); if (PageDirty(page)) goto failed; --
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