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mm: m(un)lock avoid ZERO_PAGE 

I'm still reluctant to clutter __get_user_pages() with another flag, just
to avoid touching ZERO_PAGE count in mlock(); though we can add that later
if it shows up as an issue in practice.

But when mlocking, we can test page->mapping slightly earlier, to avoid
the potentially bouncy rescheduling of lock_page on ZERO_PAGE - mlock
didn't lock_page in olden ZERO_PAGE days, so we might have regressed.

And when munlocking, it turns out that FOLL_DUMP coincidentally does
what's needed to avoid all updates to ZERO_PAGE, so use that here also.
Plus add comment suggested by KAMEZAWA Hiroyuki.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2009-09-21 17:03:32 -07:00 committed by Linus Torvalds
parent 58fa879e1e
commit 6e919717c8
1 changed files with 36 additions and 13 deletions
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49
mm/mlock.c
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@ -198,17 +198,26 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
for (i = 0; i < ret; i++) {
struct page *page = pages[i];
lock_page(page);
/*
* Because we lock page here and migration is blocked
* by the elevated reference, we need only check for
* file-cache page truncation. This page->mapping
* check also neatly skips over the ZERO_PAGE(),
* though if that's common we'd prefer not to lock it.
*/
if (page->mapping)
mlock_vma_page(page);
unlock_page(page);
if (page->mapping) {
/*
* That preliminary check is mainly to avoid
* the pointless overhead of lock_page on the
* ZERO_PAGE: which might bounce very badly if
* there is contention. However, we're still
* dirtying its cacheline with get/put_page:
* we'll add another __get_user_pages flag to
* avoid it if that case turns out to matter.
*/
lock_page(page);
/*
* 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)
mlock_vma_page(page);
unlock_page(page);
}
put_page(page); /* ref from get_user_pages() */
}
@ -309,9 +318,23 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
vma->vm_flags &= ~VM_LOCKED;
for (addr = start; addr < end; addr += PAGE_SIZE) {
struct page *page = follow_page(vma, addr, FOLL_GET);
if (page) {
struct page *page;
/*
* Although FOLL_DUMP is intended for get_dump_page(),
* it just so happens that its special treatment of the
* ZERO_PAGE (returning an error instead of doing get_page)
* suits munlock very well (and if somehow an abnormal page
* has sneaked into the range, we won't oops here: great).
*/
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);
unlock_page(page);