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xj2cZrH8O1N/ywdw+CpPOfnaECGAYSub1yPIg3MGe9ZvV6IgqNXqn7CmW99xXrKYkziAjq92h+UguVQhiTzY35s2/cJZhCCprKcjHfw27otyvb28xMYZJ7vWd9qUtHVdDtg9ZW5Pcp15sgf5QgFAeX60TZLHICaiVP0hHtwXAmeoWzqcavRTO+N21tHTrSQwbcJyR/gNUa9bkx7fACcTJPVCGuPW9H7iIDZuHRa8rPbyF8moTrFWyMCKcGReBlEliU6XwDPtU2SjS+uKNcTnqFYjlWm5rmxGokrCDhKkSy75h/n3WuL2PswGdr0JHc06NCHdzt0cFBlqk1WvUE5PBdBf2a9IWnODnbebpiCu6oC+V4cF7BGIaCYHl7AVK2qN2GCUIttXNUm/6/GEgGYmsByZ2z8Unz8JESs/HJCmexeVUBe4= 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: --=_MailMate_AAC7DC85-3691-49B8-8AE2-2FB09CD15A3D_= Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 25 Apr 2024, at 22:23, Barry Song wrote: > On Fri, Apr 26, 2024 at 9:55=E2=80=AFAM Zi Yan wrote: >> >> On 25 Apr 2024, at 21:45, Barry Song wrote: >> >>> On Fri, Apr 26, 2024 at 5:11=E2=80=AFAM Zi Yan wrot= e: >>>> >>>> From: Zi Yan >>>> >>>> In __folio_remove_rmap(), a large folio is added to deferred split l= ist >>>> if any page in a folio loses its final mapping. But it is possible t= hat >>>> the folio is fully unmapped and adding it to deferred split list is >>>> unnecessary. >>>> >>>> For PMD-mapped THPs, that was not really an issue, because removing = the >>>> last PMD mapping in the absence of PTE mappings would not have added= the >>>> folio to the deferred split queue. >>>> >>>> However, for PTE-mapped THPs, which are now more prominent due to mT= HP, >>>> they are always added to the deferred split queue. One side effect >>>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can = be >>>> unintentionally increased, making it look like there are many partia= lly >>>> mapped folios -- although the whole folio is fully unmapped stepwise= =2E >>>> >>>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THP= s >>>> where possible starting from commit b06dc281aa99 ("mm/rmap: introduc= e >>>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-m= apped >>>> folio is unmapped in one go and can avoid being added to deferred sp= lit >>>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will sti= ll be >>>> noise when we cannot batch-unmap a complete PTE-mapped folio in one = go >>>> -- or where this type of batching is not implemented yet, e.g., migr= ation. >>>> >>>> To avoid the unnecessary addition, folio->_nr_pages_mapped is checke= d >>>> to tell if the whole folio is unmapped. If the folio is already on >>>> deferred split list, it will be skipped, too. >>>> >>>> Note: commit 98046944a159 ("mm: huge_memory: add the missing >>>> folio_test_pmd_mappable() for THP split statistics") tried to exclud= e >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does = not >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was sti= ll >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,= >>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside >>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(= ). >>>> >>>> Signed-off-by: Zi Yan >>>> Reviewed-by: Yang Shi >>>> --- >>>> mm/rmap.c | 8 +++++--- >>>> 1 file changed, 5 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>> index a7913a454028..220ad8a83589 100644 >>>> --- a/mm/rmap.c >>>> +++ b/mm/rmap.c >>>> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rm= ap(struct folio *folio, >>>> * page of the folio is unmapped and at least one pa= ge >>>> * is still mapped. >>>> */ >>>> - if (folio_test_large(folio) && folio_test_anon(folio= )) >>>> - if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_p= mdmapped) >>>> - deferred_split_folio(folio); >>>> + if (folio_test_large(folio) && folio_test_anon(folio= ) && >>>> + list_empty(&folio->_deferred_list) && >>>> + ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(map= ped)) || >>>> + (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapp= ed))) >>>> + deferred_split_folio(folio); >>> >>> Hi Zi Yan, >>> in case a mTHP is mapped by two processed (forked but not CoW yet), i= f we >>> unmap the whole folio by pte level in one process only, are we still = adding this >>> folio into deferred list? >> >> No. Because the mTHP is still fully mapped by the other process. In te= rms of code, >> nr will be 0 in that case and this if condition is skipped. nr is only= increased >> from 0 when one of the subpages in the mTHP has no mapping, namely pag= e->_mapcount >> becomes negative and last is true in the case RMAP_LEVEL_PTE. > > Ok. i see, so "last" won't be true? > > case RMAP_LEVEL_PTE: > do { > last =3D atomic_add_negative(-1, &page->_mapcount); > if (last && folio_test_large(folio)) { > last =3D atomic_dec_return_relaxed(mapped); > last =3D (last < ENTIRELY_MAPPED); > } > > if (last) > nr++; > } while (page++, --nr_pages > 0); > break; Right, because for every subpage its corresponding last =3D atomic_add_negative(-1, &page->_mapcount); is not true after the= unmapping. -- Best Regards, Yan, Zi --=_MailMate_AAC7DC85-3691-49B8-8AE2-2FB09CD15A3D_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmYrFmwPHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhUm4UQAJ0aWBln5/8UqROLl4+DY5ruik7J6kqv5U4e sRutUToZX07er2ou/uaYXzq7fb0jsvrqOZuA0StrKsE3YeKQ9DyILAoPEv97f3j6 QkNkpMzmgdJ69LGeohjBnIP5WOE7wyE+dwmMgNtITd9W6GDj5U5g//EKVs8ufvuG 27yq672HvNzFIEVVsTD+vjT9nVenyymzg93NW/MaU4esmHeur5DNvF63qavuGr5D RI6MmFZSeASfDf76rsUEJrYYUpdzyCmlNIhuynDOe1+WV2RZXDtT8cvoDTqp2X8l J1VYYABTL5u/TbQOgOiU6daua5pLtX2fZ1SkrM+U4WDVvzrsErTtmUDjzZGEEiS3 x1f05ODyBNoMWhW9ov3S2GmjPqFqLQrMhcJ5w4GE+SmdtrAkG2s7lt/+tL1NHh/R ZJAYjiMfxZI+Xa4e0TqBIGJ3PyOLLMjZAEGdcHpPDqNj6TDFD04t3vsFLUGTMKNv VqIPMxb3tgjRN/CnwkcsKvZFMrBeT0Uxh5IfxreVO09gcThoFrvIdT9WprJq5zi5 tELq1NtbU23fXA2tljYhtDRD4IN41E53pQJ2aW0x2TslWzfiqZIJn1LMQ12E0A8j 8B0BEkVd84E0dbSUaw/j02jwWSJ+U2kMtcpjV9gdigorFN82hKvMgL9ai10xXw0X oLAXNonZ =457H -----END PGP SIGNATURE----- --=_MailMate_AAC7DC85-3691-49B8-8AE2-2FB09CD15A3D_=--