linux-kernel - Re: [PATCH] mm/vma: Do not register private-anon mappings with khugepaged during mmap

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Message-ID: <c8eb5701-47ad-4cca-b123-31a5a978f801@arm.com>
Date: Thu, 6 Mar 2025 20:02:10 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, Liam.Howlett@...cle.com, vbabka@...e.cz,
 jannh@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 ryan.roberts@....com, anshuman.khandual@....com, aneesh.kumar@...nel.org,
 yang@...amperecomputing.com, david@...hat.com, willy@...radead.org,
 hughd@...gle.com, ziy@...dia.com
Subject: Re: [PATCH] mm/vma: Do not register private-anon mappings with
 khugepaged during mmap



On 06/03/25 1:48 pm, Lorenzo Stoakes wrote:
> On Thu, Mar 06, 2025 at 12:00:37PM +0530, Dev Jain wrote:
>> We already are registering private-anon VMAs with khugepaged during fault
>> time, in do_huge_pmd_anonymous_page(). Commit "register suitable readonly
>> file vmas for khugepaged" moved the khugepaged registration logic from
>> shmem_mmap to the generic mmap path. Make this logic specific for non-anon
>> mappings.
> 
> This does sound reasonable, thanks! Though does need to be expanded as
> Andrew says for user-visible effects just to be crystal clear.

Sure.

> 
>>
>> Fixes: 613bec092fe7 ("mm: mmap: register suitable readonly file vmas for khugepaged")
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   mm/vma.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vma.c b/mm/vma.c
>> index af1d549b179c..730a26bf14a5 100644
>> --- a/mm/vma.c
>> +++ b/mm/vma.c
>> @@ -2377,7 +2377,8 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>>   	 * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
>>   	 * call covers the non-merge case.
>>   	 */
>> -	khugepaged_enter_vma(vma, map->flags);
>> +	if (!vma_is_anonymous(vma))
>> +		khugepaged_enter_vma(vma, map->flags);
> 
> This really needs a comment :) as a Joe Bloggs coder coming to this my
> immediate thought would be 'huh? Why?'
> 
> Something like:
> 
> 	/* Just added so khugepaged has nothing to do. We call again on fault. */
> 
> Would be great.
> 
> Thanks!

Sure.

BTW does this patch merit a CC:stable? I am not aware of the general 
practice but I am not sure if this is a *serious bug*, as per 
submitting-patches.rst.

> 
> 
>>   	ksm_add_vma(vma);
>>   	*vmap = vma;
>>   	return 0;
>> --
>> 2.30.2
>>