Re: [PATCH v2 00/12] mm: page migration enhancement for thp

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Wed, Nov 09, 2016 at 01:32:04PM +1100, Balbir Singh wrote:
> On 08/11/16 10:31, Naoya Horiguchi wrote:
> > Hi everyone,
> > 
> > I've updated thp migration patches for v4.9-rc2-mmotm-2016-10-27-18-27
> > with feedbacks for ver.1.
> > 
> > General description (no change since ver.1)
> > ===========================================
> > 
> > This patchset enhances page migration functionality to handle thp migration
> > for various page migration's callers:
> >  - mbind(2)
> >  - move_pages(2)
> >  - migrate_pages(2)
> >  - cgroup/cpuset migration
> >  - memory hotremove
> >  - soft offline
> > 
> > The main benefit is that we can avoid unnecessary thp splits, which helps us
> > avoid performance decrease when your applications handles NUMA optimization on
> > their own.
> > 
> > The implementation is similar to that of normal page migration, the key point
> > is that we modify a pmd to a pmd migration entry in swap-entry like format.
> > 
> > Changes / Notes
> > ===============
> > 
> > - pmd_present() in x86 checks _PAGE_PRESENT, _PAGE_PROTNONE and _PAGE_PSE
> >   bits together, which makes implementing thp migration a bit hard because
> >   _PAGE_PSE bit is currently used by soft-dirty in swap-entry format.
> >   I was advised to dropping _PAGE_PSE in pmd_present(), but I don't think
> >   of the justification, so I keep it in this version. Instead, my approach
> >   is to move _PAGE_SWP_SOFT_DIRTY to bit 6 (unused) and reserve bit 7 for
> >   pmd non-present cases.
> 
> Thanks, IIRC
> 
> pmd_present = _PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE
> 
> AutoNUMA balancing would change it to
> 
> pmd_present = _PAGE_PROTNONE | _PAGE_PSE
> 
> and PMD_SWP_SOFT_DIRTY would make it
> 
> pmd_present = _PAGE_PSE
> 
> What you seem to be suggesting in your comment is that
> 
> pmd_present should be _PAGE_PRESENT | _PAGE_PROTNONE

This (no _PAGE_PSE) was a possibile solution, and as I described I gave up
this solution, because I noticed that what I actually wanted was that
pmd_present() certainly returns false during thp migration and that's done
by moving _PAGE_SWP_SOFT_DIRTY. So

  pmd_present = _PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE

is still correct in this patchset.

> 
> Isn't that good enough?
> 
> For THP migration I guess we use
> 
> _PAGE_PRESENT | _PAGE_PROTNONE | is_migration_entry(pmd)

Though I might misread your notations, I hope that the following code
seems describe itself well.

  static inline int is_pmd_migration_entry(pmd_t pmd)                            
  {                                                                              
          return !pmd_present(pmd) && is_migration_entry(pmd_to_swp_entry(pmd)); 
  }                                                                              

Thanks,
Naoya Horiguchi

> 
> 
> > 
> > - this patchset still covers only x86_64. Zi Yan posted a patch for ppc64
> >   and I think it's favorably received so that's fine. But there's unsolved
> >   minor suggestion by Aneesh, so I don't include it in this set, expecting
> >   that it will be updated/reposted.
> > 
> > - pte-mapped thp and doubly-mapped thp were not supported in ver.1, but
> >   this version should work for such kinds of thp.
> > 
> > - thp page cache is not tested yet, and it's at the head of my todo list
> >   for future version.
> > 
> > Any comments or advices are welcomed.
> 
> Balbir Singh
>