Re: [RFC PATCH 07/14] mm/khugepaged: add vm_flags_ignore to hugepage_vma_revalidate_pmd_count()

[Michal Hocko <mhocko@suse.com>]

[Dropped Richard Henderson from the CC list as the delivery fails for
 him]

On Thu 10-03-22 10:54:03, David Rientjes wrote:
> On Thu, 10 Mar 2022, David Hildenbrand wrote:
> 
> > On 08.03.22 22:34, Zach O'Keefe wrote:
> > > In madvise collapse context, we optionally want to be able to ignore
> > > advice from MADV_NOHUGEPAGE-marked regions.
> > > 
> > > Add a vm_flags_ignore argument to hugepage_vma_revalidate_pmd_count()
> > > which can be used to ignore vm flags used when considering thp
> > > eligibility.
> > 
> > arch/s390/mm/gmap.c:thp_split_mm() sets VM_NOHUGEPAGE to make sure there
> > are *really* no thp. Being able to bypass that would break KVM horribly.
> > 
> > Ignoring MADV_NOHUGEPAGE/VM_NOHUGEPAGE feels like the wrong way to go.
> > 
> 
> Agreed, we'll have to remove this possibility.

yeah, this sounds like a bug to me.

> > What about a prctl instead, to disable any khugepagd activity and just
> > let that process control it manually?
> > 
> 
> No objection to the prctl, although it's unfortunate that the existing 
> PR_SET_THP_DISABLE simply disables thp for the process entirely for any 
> non-zero value and that this wasn't implemented as a bitmask to specify 
> future behavior where this new behavior could be defined :/

I do not think PR_SET_THP_DISABLE is any different from VM_NOHUGEPAGE.
The process (owner) has opeted out from THPs for different reasons.
Those might be unknown to whoever calls the madvise call (including
itself).
-- 
Michal Hocko
SUSE Labs