Re: [patch] mm: unlockless reclaim

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 12 Jul 2007 06:11:15 +0200
Nick Piggin <npiggin@suse.de> wrote:

> unlock_page is pretty expensive. Even after my patches to optimise the
> memory order and away the waitqueue hit for uncontended pages, it is
> still a locked operation, which may be anywhere up to hundreds of cycles
> on some CPUs.
> 
> When we reclaim a page, we don't need to "unlock" it as such, because
> we know there will be no contention (if there was, it would be a bug
> because the page is just about to get freed).
> 
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> 
> Index: linux-2.6/include/linux/page-flags.h
> ===================================================================
> --- linux-2.6.orig/include/linux/page-flags.h
> +++ linux-2.6/include/linux/page-flags.h
> @@ -115,6 +115,8 @@
>  		test_and_set_bit(PG_locked, &(page)->flags)
>  #define ClearPageLocked(page)		\
>  		clear_bit(PG_locked, &(page)->flags)
> +#define __ClearPageLocked(page)		\
> +		__clear_bit(PG_locked, &(page)->flags)
>  #define TestClearPageLocked(page)	\
>  		test_and_clear_bit(PG_locked, &(page)->flags)
>  
> Index: linux-2.6/mm/vmscan.c
> ===================================================================
> --- linux-2.6.orig/mm/vmscan.c
> +++ linux-2.6/mm/vmscan.c
> @@ -576,7 +576,7 @@ static unsigned long shrink_page_list(st
>  			goto keep_locked;
>  
>  free_it:
> -		unlock_page(page);
> +		__ClearPageLocked(page);
>  		nr_reclaimed++;
>  		if (!pagevec_add(&freed_pvec, page))
>  			__pagevec_release_nonlru(&freed_pvec);

I really hate this patch :(  For the usual reasons.

I'd have thought that such a terrifying point-cannon-at-someone-else's-foot
hack would at least merit a comment explaining (fully) to the reader why it
is a safe thing to do at this site.

And explaining to them why __pagevec_release_nonlru() immediately
contradicts the assumption which this code is making.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>