Ryan Roberts <ryan.roberts@xxxxxxx> writes: > On 10/07/2023 06:37, Huang, Ying wrote: >> Ryan Roberts <ryan.roberts@xxxxxxx> writes: >> >>> Somehow I managed to reply only to the linux-arm-kernel list on first attempt so >>> resending: >>> >>> On 07/07/2023 09:21, Huang, Ying wrote: >>>> Ryan Roberts <ryan.roberts@xxxxxxx> writes: >>>> >>>>> With the introduction of large folios for anonymous memory, we would >>>>> like to be able to split them when they have unmapped subpages, in order >>>>> to free those unused pages under memory pressure. So remove the >>>>> artificial requirement that the large folio needed to be at least >>>>> PMD-sized. >>>>> >>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> >>>>> Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx> >>>>> Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx> >>>>> --- >>>>> mm/rmap.c | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>> index 82ef5ba363d1..bbcb2308a1c5 100644 >>>>> --- a/mm/rmap.c >>>>> +++ b/mm/rmap.c >>>>> @@ -1474,7 +1474,7 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, >>>>> * page of the folio is unmapped and at least one page >>>>> * is still mapped. >>>>> */ >>>>> - if (folio_test_pmd_mappable(folio) && folio_test_anon(folio)) >>>>> + if (folio_test_large(folio) && folio_test_anon(folio)) >>>>> if (!compound || nr < nr_pmdmapped) >>>>> deferred_split_folio(folio); >>>>> } >>>> >>>> One possible issue is that even for large folios mapped only in one >>>> process, in zap_pte_range(), we will always call deferred_split_folio() >>>> unnecessarily before freeing a large folio. >>> >>> Hi Huang, thanks for reviewing! >>> >>> I have a patch that solves this problem by determining a range of ptes covered >>> by a single folio and doing a "batch zap". This prevents the need to add the >>> folio to the deferred split queue, only to remove it again shortly afterwards. >>> This reduces lock contention and I can measure a performance improvement for the >>> kernel compilation benchmark. See [1]. >>> >>> However, I decided to remove it from this patch set on Yu Zhao's advice. We are >>> aiming for the minimal patch set to start with and wanted to focus people on >>> that. I intend to submit it separately later on. >>> >>> [1] https://lore.kernel.org/linux-mm/20230626171430.3167004-8-ryan.roberts@xxxxxxx/ >> >> Thanks for your information! "batch zap" can solve the problem. >> >> And, I agree with Matthew's comments to fix the large folios interaction >> issues before merging the patches to allocate large folios as in the >> following email. >> >> https://lore.kernel.org/linux-mm/ZKVdUDuwNWDUCWc5@xxxxxxxxxxxxxxxxxxxx/ >> >> If so, we don't need to introduce the above problem or a large patchset. > > I appreciate Matthew's and others position about not wanting to merge a minimal > implementation while there are some fundamental features (e.g. compaction) it > doesn't play well with - I'm working to create a definitive list so these items > can be tracked and tackled. Good to know this, Thanks! > That said, I don't see this "batch zap" patch as an example of this. It's just a > performance enhancement that improves things even further than large anon folios > on their own. I'd rather concentrate on the core changes first then deal with > this type of thing later. Does that work for you? IIUC, allocating large folios upon page fault depends on splitting large folios in page_remove_rmap() to avoid memory wastage. Splitting large folios in page_remove_rmap() depends on "batch zap" to avoid performance regression in zap_pte_range(). So we need them to be done earlier. Or I miss something? Best Regards, Huang, Ying
