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

[David Hildenbrand]

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@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?


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