linux-kernel - Re: [RFC 1/3] mm/ksm: add anonymous check in find_mergeable

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Date: Fri, 7 Jun 2024 12:13:16 +0200
From: David Hildenbrand <david@...hat.com>
To: Alex Shi <seakeel@...il.com>, alexs@...nel.org,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, izik.eidus@...ellosystems.com,
 willy@...radead.org, aarcange@...hat.com, chrisw@...s-sol.org,
 hughd@...gle.com
Subject: Re: [RFC 1/3] mm/ksm: add anonymous check in find_mergeable_vma

On 07.06.24 11:33, Alex Shi wrote:
> 
> 
> On 6/5/24 5:56 PM, David Hildenbrand wrote:
>> On 05.06.24 11:53, alexs@...nel.org wrote:
>>> From: "Alex Shi (tencent)" <alexs@...nel.org>
>>>
>>> We do vma_set_anonyous in do_mmap(), and then vma_is_anonymous()
>>> checking workable, use it as a extra check since ksm only care anonymous
>>> pages.
>>>
>>> Signed-off-by: Alex Shi (tencent) <alexs@...nel.org>
>>> ---
>>>    mm/ksm.c | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index f5138f43f0d2..088bce39cd33 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -742,7 +742,8 @@ static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
>>>        if (ksm_test_exit(mm))
>>>            return NULL;
>>>        vma = vma_lookup(mm, addr);
>>> -    if (!vma || !(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
>>> +    if (!vma || !(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma ||
>>> +            !vma_is_anonymous(vma))
>>
>> Doesn't KSM also apply to COW'ed pages in !anon mappings? At least that's what I recall.
> I didn't a evidence for this. :(
> 
> In write_protect_page(), "PageAnonExclusive(&folio->page);" has a "VM_BUG_ON_PGFLAGS(!PageAnon(page), page);"
> So is this hints the vma also need to be anonymous one?


vma_is_anonymous(vma) is restricted to anonymous folios and the shared 
zeropage, but other VMAs (MAP_PRIVATE file-backed VMAs) can contain 
anonymous folios as well.

-- 
Cheers,

David / dhildenb