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 ZNu+GdFM QMTDUd3JXvWgXoh1AOdxLDXjUaIkzR0yaDmKTjzLGFfeSAUqpn01zslRpi3Xv65AXK7LvWQe6uGjzQGvfBHwlldbGEKJdv0aYvb+hOyWs6+ykCNsqonw7U7/1FX1doqbV/sC4KnaRthqRuOB4HkNs4EOuvOoib1mdW1ATykrjiRw0vBrthuBhZgoWMYwHpuPePSxht5pM8q+9DOE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 在 2025/7/24 17:45, Lorenzo Stoakes 写道: > On Thu, Jul 24, 2025 at 05:36:27PM +0800, Huan Yang wrote: >> 在 2025/7/24 17:32, David Hildenbrand 写道: >>> On 24.07.25 11:20, David Hildenbrand wrote: >>>> On 24.07.25 11:12, David Hildenbrand wrote: >>>>> On 24.07.25 11:09, Huan Yang wrote: >>>>>> 在 2025/7/24 16:59, David Hildenbrand 写道: >>>>>>> On 24.07.25 10:44, Huan Yang wrote: >>>>>>>> Summary >>>>>>>> == >>>>>>>> This patchset reuses page_type to store migrate >>>>>>>> entry count during the >>>>>>>> period from migrate entry setup to removal, enabling accelerated VMA >>>>>>>> traversal when removing migrate entries, following a similar >>>>>>>> principle to >>>>>>>> early termination when folio is unmapped in try_to_migrate. >>>>>>> I absolutely detest (ab)using page types for that, so no from my side >>>>>>> unless I am missing something important. >>>>>>> >>>>>>>> In my self-constructed test scenario, the migration >>>>>>>> time can be reduced >>>>>>> How relevant is that in practice? >>>>>> IMO, any folio mapped < nr vma in mapping(anon_vma, addresss_space), >>>>>> will benefit from this. >>>>>> >>>>>> So, all pages that have been COW-ed by child processes can be skipped. >>>>> For small anon folios, you could use the anon-exclusive marker >>>>> to derive >>>>> "there can only be a single mapping". >>>>> >>>>> It's stored alongside the migration entry. >>>>> >>>>> So once you restored that single migration entry, you can just stop the >>>>> walk. >>>> Essentially, something (untested) like this: >>>> >>>> diff --git a/mm/migrate.c b/mm/migrate.c >>>> index 425401b2d4e14..aa5bf96b1daee 100644 >>>> --- a/mm/migrate.c >>>> +++ b/mm/migrate.c >>>> @@ -421,6 +421,15 @@ static bool remove_migration_pte(struct folio >>>> *folio, >>>>                      /* No need to invalidate - it was non-present >>>> before */ >>>>                   update_mmu_cache(vma, pvmw.address, pvmw.pte); >>>> + >>>> +               /* >>>> +                * If the small anon folio is exclusive, here can be >>>> exactly one >>>> +                * page mapping -- the one we just restored. >>>> +                */ >>>> +               if (!folio_test_large(folio) && (rmap_flags & >>>> RMAP_EXCLUSIVE)) { >>>> +                       page_vma_mapped_walk_done(&pvmw); >>>> +                       break; >>>> +               } >>>>           } >>>>              return true; >>> Probably that won't really help I assume, because __folio_set_anon() >>> will move the new anon folio under vma->anon_vma, not >>> vma->anon_vma->root. >>> >>> So I assume you mean that we had a COW-shared folio now mapped only into >>> some VMAs (some mappings in other processes removed due to CoW or >>> similar). >>> >>> In that case aborting early can help. >>> >>> Not in all cases though, just imagine that the very last VMA we're >>> iterating maps the page. You have to iterate through all of them either >>> way ... no way around that, really. >> Indeed, whether we can exit the loop early depends on the position of the >> terminating VMA in the tree. >> >> I think a better approach would be to remove the fully COW-ed VMAs and their >> associated AVCs from the anon_vma's tree. >> >> I've been researching this aspect, but haven't made any progress yet.(I have >> some ideas, but the specific implementation is still challenging.) >> > Please leave this alone, I'm in the midst of trying to make fundamental changes > to the anon rmap logic and it's really very subtle and indeed challenging (as > you've seen). OK, but I still try to research it, not only for upstream or something. > > Since I intend to change the whole mechanism around this, efforts to adjust the > existing behaviour are going to strictly conflict with that. > > We are 'lazy' in actually properly accounting for fully CoW'd VMAs and so can > only know 'maybe' if it has, I mean as from above you've noticed. > > The CoW hierarchy also makes life challenging, see vma_had_uncowed_parents() for > an example of the subtlty. > > Having looked at anon rmap in detail, I have come to think the only sensible way > forward is something fairly bold. Woo, that's really interesting. I'm looking forward to this. Thanks. > > Thanks! >