Re: [PATCH v3 2/2] mm: shmem: disallow hugepages if the system-wide shmem THP sysfs settings are disabled

[Baolin Wang]

On 2025/6/23 21:59, David Hildenbrand wrote:
> On 23.06.25 10:28, Baolin Wang wrote:
> > When invoking thp_vma_allowable_orders(), if the TVA_ENFORCE_SYSFS 
> > flag is not
> > specified, we will ignore the THP sysfs settings. And the 
> > MADV_COLLAPSE is an
> > example of such a case.
> >
> > The MADV_COLLAPSE will ignore the system-wide shmem THP sysfs 
> > settings, which
> > means that even though we have disabled the shmem THP configuration, 
> > MADV_COLLAPSE
> > will still attempt to collapse into a shmem THP. This violates the 
> > rule we have
> > agreed upon: never means never.
> >
> > Another rule for madvise, referring to David's suggestion: “allowing 
> > for collapsing
> > in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
> >
> > To fix the MADV_COLLAPSE issue for shmem, then the current strategy 
> > should be:
> >
> > For shmem, if none of always, madvise, within_size, and inherit have 
> > enabled
> > PMD-sized THP, then MADV_COLLAPSE will be prohibited from collapsing 
> > PMD-sized THP.
>
> I assume we could rephrase that to "For shmem, if "shmem_enabled" is set 
> to either "none" or "deny", then MADV_COLLAPSE will be prohibited from 
> collapsing."

Yes. Setting 'deny' will also prevent MADV_COLLAPSE(), and there is no 
'none' option for 'shmem_enabled'. Will update the commit mesasge to 
make it clear.