Messages in this thread | | | Date | Sat, 20 Apr 2024 18:31:58 +0200 | Subject | Re: [PATCH 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list() | From | David Hildenbrand <> |
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On 20.04.24 17:04, Lance Yang wrote: > On Sat, Apr 20, 2024 at 12:59 PM Lance Yang <ioworker0@gmail.com> wrote: >> >> Hey Matthew, >> >> Thanks for taking time to review! >> >> On Wed, Apr 17, 2024 at 11:09 PM Matthew Wilcox <willy@infradead.org> 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@gmail.com> >>>> --- >>>> 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
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