Re: [RFC 1/3] mm/ksm: add anonymous check in find_mergeable_vma

[Alex Shi]

On 6/5/24 5:56 PM, David Hildenbrand wrote:
> On 05.06.24 11:53, alexs@xxxxxxxxxx wrote:
>> From: "Alex Shi (tencent)" <alexs@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> ---
>>   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?


Thanks a lot!
Alex
>