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

[David Hildenbrand]

On 20.04.24 17:04, Lance Yang wrote:
> 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.

You must follow the exact same logic as in try_to_unmap_one() I guess.

That is, unmap the page, syncing against concurrent GUP-fast. Then, 
check mapcount vs. refcount. If there are unexpected references, remap 
the page (set_pte_at).

--
Cheers,

David / dhildenb