Re: [PATCH 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list()

[Lance Yang]

On Sat, Apr 20, 2024 at 12:59 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
>
> Hey Matthew,
>
> Thanks for taking time to review!
>
> On Wed, Apr 17, 2024 at 11:09 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, Apr 17, 2024 at 10:11:11PM +0800, Lance Yang wrote:
> > > When the user no longer requires the pages, they would use madvise(madv_free)
> > > to mark the pages as lazy free. IMO, they would not typically rewrite to the
> > > given range.
> > >
> > > At present, a PMD-mapped THP marked as lazyfree during shrink_folio_list()
> > > is unconditionally split, which may be unnecessary. If the THP is exclusively
> > > mapped and clean, and the PMD associated with it is also clean, then we can
> > > attempt to remove the PMD mapping from it. This change will improve the
> > > efficiency of memory reclamation in this case.
> > >
> > > On an Intel i5 CPU, reclaiming 1GiB of PMD-mapped THPs using
> > > mem_cgroup_force_empty() results in the following runtimes in seconds
> > > (shorter is better):
> > >
> > > --------------------------------------------
> > > |     Old       |      New       |  Change  |
> > > --------------------------------------------
> > > |   0.683426    |    0.049197    |  -92.80% |
> > > --------------------------------------------
> > >
> > > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > > ---
> > >  include/linux/huge_mm.h |  1 +
> > >  include/linux/rmap.h    |  1 +
> > >  mm/huge_memory.c        |  2 +-
> > >  mm/rmap.c               | 81 +++++++++++++++++++++++++++++++++++++++++
> > >  mm/vmscan.c             |  7 ++++
> > >  5 files changed, 91 insertions(+), 1 deletion(-)
> >
> > I'm confused why we need all this extra code.  If we remove a folio
>
> Thanks for pointing that out!
>
> I've added a lot of extra code to rmap.c, and we don't need it
> for file pages - sorry. I'll reconsider where to place this code.
>
> > from the pagecache, we can just call truncate_inode_folio() and
> > unmap_mapping_folio() takes care of all the necessary unmappings.
> > Why can't you call unmap_mapping_folio() here?
>
> Thanks for your suggestion.
>
> But this change only avoids the splitting of *anon* large folios
> (PMD-mapped THPs) that are marked as lazyfree during
> shrink_folio_list().
>
> IIUC, in some cases, we cannot unmap the THP marked as lazyfree
> here, such as when it's not exclusively mapped, dirty, pinned, etc.

I’d like to make a correction.

IMO, we can unmap the THP that is not exclusively mapped, but
ensuring folio_ref_count() equals folio_mapcount() +1.

Thanks,
Lance

> In such situations, we still need to return to try_to_unmap_one(), and
> then call split_huge_pmd_address() to split it.
>
> Thanks again for the review.
> Lance