linux-kernel - Re: [PATCH] mm: incorporate read-only pages into transparent huge pages

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Date:	Fri, 23 Jan 2015 17:58:02 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Rik van Riel <riel@...hat.com>
Cc:	Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, mhocko@...e.cz, mgorman@...e.de,
	rientjes@...gle.com, sasha.levin@...cle.com, hughd@...gle.com,
	hannes@...xchg.org, vbabka@...e.cz, linux-kernel@...r.kernel.org,
	aarcange@...hat.com
Subject: Re: [PATCH] mm: incorporate read-only pages into transparent huge
 pages

On Fri, Jan 23, 2015 at 09:57:03AM -0500, Rik van Riel wrote:
> On 01/23/2015 06:37 AM, Kirill A. Shutemov wrote:
> > On Fri, Jan 23, 2015 at 09:47:36AM +0200, Ebru Akagunduz wrote:
> >> This patch aims to improve THP collapse rates, by allowing
> >> THP collapse in the presence of read-only ptes, like those
> >> left in place by do_swap_page after a read fault.
> >>
> >> Currently THP can collapse 4kB pages into a THP when
> >> there are up to khugepaged_max_ptes_none pte_none ptes
> >> in a 2MB range. This patch applies the same limit for
> >> read-only ptes.
> 
> >> @@ -2179,6 +2179,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >>  		 */
> >>  		if (!trylock_page(page))
> >>  			goto out;
> >> +		if (!pte_write(pteval)) {
> >> +			if (PageSwapCache(page) && !reuse_swap_page(page)) {
> >> +					unlock_page(page);
> >> +					goto out;
> >> +			}
> >> +			/*
> >> +			 * Page is not in the swap cache, and page count is
> >> +			 * one (see above). It can be collapsed into a THP.
> >> +			 */
> >> +		}
> > 
> > Hm. As a side effect it will effectevely allow collapse in PROT_READ vmas,
> > right? I'm not convinced it's a good idea.
> 
> It will only allow a THP collapse if there is at least one
> read-write pte.
> 
> I suspect that excludes read-only VMAs automatically.

Ah. Okay. I missed that condition.

Looks good to me.

-- 
 Kirill A. Shutemov
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