linux-kernel - Re: [PATCH 3/5] mm: clear PG

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Date:	Tue, 27 Oct 2015 15:50:32 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 3/5] mm: clear PG_dirty to mark page freeable

On Mon, Oct 26, 2015 at 06:28:13PM -0700, Hugh Dickins wrote:
> On Mon, 19 Oct 2015, Minchan Kim wrote:
> 
> > Basically, MADV_FREE relies on dirty bit in page table entry
> > to decide whether VM allows to discard the page or not.
> > IOW, if page table entry includes marked dirty bit, VM shouldn't
> > discard the page.
> > 
> > However, as a example, if swap-in by read fault happens,
> > page table entry doesn't have dirty bit so MADV_FREE could discard
> > the page wrongly.
> > 
> > For avoiding the problem, MADV_FREE did more checks with PageDirty
> > and PageSwapCache. It worked out because swapped-in page lives on
> > swap cache and since it is evicted from the swap cache, the page has
> > PG_dirty flag. So both page flags check effectively prevent
> > wrong discarding by MADV_FREE.
> > 
> > However, a problem in above logic is that swapped-in page has
> > PG_dirty still after they are removed from swap cache so VM cannot
> > consider the page as freeable any more even if madvise_free is
> > called in future.
> > 
> > Look at below example for detail.
> > 
> >     ptr = malloc();
> >     memset(ptr);
> >     ..
> >     ..
> >     .. heavy memory pressure so all of pages are swapped out
> >     ..
> >     ..
> >     var = *ptr; -> a page swapped-in and could be removed from
> >                    swapcache. Then, page table doesn't mark
> >                    dirty bit and page descriptor includes PG_dirty
> >     ..
> >     ..
> >     madvise_free(ptr); -> It doesn't clear PG_dirty of the page.
> >     ..
> >     ..
> >     ..
> >     .. heavy memory pressure again.
> >     .. In this time, VM cannot discard the page because the page
> >     .. has *PG_dirty*
> > 
> > To solve the problem, this patch clears PG_dirty if only the page
> > is owned exclusively by current process when madvise is called
> > because PG_dirty represents ptes's dirtiness in several processes
> > so we could clear it only if we own it exclusively.
> > 
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> 
> Acked-by: Hugh Dickins <hughd@...gle.com>
> 
> (and patches 1/5 and 2/5 too if you like)
> 

Thanks for the review, Hugh!

I will rebase all series from the beginning as you suggested
and will add your Acked-by because I feel you just reviewed
all of MADV_FREE code line and you have no found any problem.

If something happens(ie, I abuse your Acked-by) wrong, please
shout me.
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