Re: [PATCH 1/4] mm: m(un)lock avoid ZERO_PAGE

[Mel Gorman <mel@csn.ul.ie>]

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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