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SubjectRe: [PATCH 1/4] mm: m(un)lock avoid ZERO_PAGE
On Tue, Sep 15, 2009 at 09:31:49PM +0100, Hugh Dickins wrote:
> 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>
> ---
>
> mm/mlock.c | 49 ++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 36 insertions(+), 13 deletions(-)
>
> --- mm0/mm/mlock.c 2009-09-14 16:34:37.000000000 +0100
> +++ mm1/mm/mlock.c 2009-09-15 17:32:03.000000000 +0100
> @@ -198,17 +198,26 @@ static long __mlock_vma_pages_range(stru
> 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_a
> 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);

Ouch, now I get your depraved comment :) . This will be a tricky rule to
remember in a years time, wouldn't it?

> + 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);
>

Functionally, the patch seems fine and the avoidance of lock_page() is
nice so.

Reviewed-by: Mel Gorman <mel@csn.ul.ie>

But, as FOLL_DUMP applies to more than core dumping, can it be renamed
in another follow-on patch? The fundamental underlying "thing" it does
is to error instead of faulting the zero page so FOLL_NO_FAULT_ZEROPAGE,
FOLL_ERRORZERO, FOLL_NOZERO etc? A name like that would simplify the comments
as FOLL_DUMP would no longer just be a desirable side-effect.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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