linux-kernel - Re: [PATCH 02/10] mm/ksm: skip subpages of compound pages

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Date: Wed, 5 Jun 2024 14:14:13 +0800
From: Alex Shi <seakeel@...il.com>
To: Matthew Wilcox <willy@...radead.org>, alexs@...nel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, izik.eidus@...ellosystems.com,
 aarcange@...hat.com, chrisw@...s-sol.org, hughd@...gle.com, david@...hat.com
Subject: Re: [PATCH 02/10] mm/ksm: skip subpages of compound pages



On 6/5/24 11:52 AM, Matthew Wilcox wrote:
> On Tue, Jun 04, 2024 at 12:24:44PM +0800, alexs@...nel.org wrote:
>> From: "Alex Shi (tencent)" <alexs@...nel.org>
>>
>> When a folio isn't fit for KSM, the subpages are unlikely to be good,
>> So let's skip the rest page checking to save some actions.
> 
> Why would you say that is true?  We have plenty of evidence that
> userspace allocators can allocate large folios, then use only the first
> few bytes, leaving many tail pages full of zeroes.

Um, that do need tail pages... 
Is there some way to use more folio in ksm?

> 
>> @@ -2660,6 +2662,9 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>>  				cond_resched();
>>  				continue;
>>  			}
>> +
>> +			VM_WARN_ON(PageTail(*page));
>> +			nr = compound_nr(*page);
> 
> And this is simply wrong.  *page can obviously be a tail page.
> 

Got it. Thanks a lot!
Alex