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

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Date:	Fri, 23 Jan 2015 09:57:03 -0500
From:	Rik van Riel <riel@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Ebru Akagunduz <ebru.akagunduz@...il.com>
CC:	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 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.
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