Re: [v3 PATCH 0/8] Make khugepaged collapse readonly FS THP more consistent

[Yang Shi]

On Wed, Apr 27, 2022 at 1:59 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Apr 04, 2022 at 05:48:49PM -0700, Yang Shi wrote:
> > When khugepaged collapses file THPs, its behavior is not consistent.
> > It is kind of "random luck" for khugepaged to see the file vmas (see
> > report: https://lore.kernel.org/linux-mm/00f195d4-d039-3cf2-d3a1-a2c88de397a0@xxxxxxx/)
> > since currently the vmas are registered to khugepaged when:
> >   - Anon huge pmd page fault
> >   - VMA merge
> >   - MADV_HUGEPAGE
> >   - Shmem mmap
> >
> > If the above conditions are not met, even though khugepaged is enabled
> > it won't see any file vma at all.  MADV_HUGEPAGE could be specified
> > explicitly to tell khugepaged to collapse this area, but when
> > khugepaged mode is "always" it should scan suitable vmas as long as
> > VM_NOHUGEPAGE is not set.
>
> I don't see that as being true at all.  The point of this hack was that
> applications which really knew what they were doing could enable it.
> It makes no sense to me that setting "always" by the sysadmin for shmem
> also force-enables ROTHP, even for applications which aren't aware of it.
>
> Most telling, I think, is that Song Liu hasn't weighed in on this at
> all.  It's clearly not important to the original author.

Song Liu already acked the series, please see
https://lore.kernel.org/linux-mm/96F2D93B-2043-44C3-8062-C639372A0212@xxxxxx/