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/3c66am+4XsNKQAqzP5vDPpMSBCK3k9YKFwG7B4mjYROBZI/wqlNpTlf5+ADgUXCNQDQXD7g9awYocWnCUr47Sxl/kywi+zcGTNzYpGYQpPd36Y6ZRR6edd4qAyl+93xKbzpsRUn6ULxisoaDi4aQWbs6lzD5SxXOcDnq9Hmd8cDBE+ZCxqNXQFlHrJ/mq4LY8ioYvBBn7Uu7poCPemmrR0NXmkmbg6X4n/aE9j4C6bLsmeuG2NsUV9LcefL7Nl9dCm5koAm3AIyVIU4bQKSlcT8ELn3F+jxt2rydQV6mIeIKcCFiOjaTWyGy4wxJ83jYpOWpTAOzAuEVo+2kiXA/2OgkE2KyKNIEGIaJ2uG1/QNgJYqrw61IOGdNXVZ2qVifyO6qX1TLeILmp46pS8dEUMm8WANzGOYpS5IQQ+To3x4YHEE= 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_5B2280AB-7499-4B5A-95DA-0149144A2252_= Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 25 Apr 2024, at 21:45, Barry Song wrote: > On Fri, Apr 26, 2024 at 5:11=E2=80=AFAM Zi Yan wrote:= >> >> From: Zi Yan >> >> In __folio_remove_rmap(), a large folio is added to deferred split lis= t >> if any page in a folio loses its final mapping. But it is possible tha= t >> 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 th= e >> last PMD mapping in the absence of PTE mappings would not have added t= he >> folio to the deferred split queue. >> >> However, for PTE-mapped THPs, which are now more prominent due to mTHP= , >> 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 partiall= y >> mapped folios -- although the whole folio is fully unmapped stepwise. >> >> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs >> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce >> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-map= ped >> folio is unmapped in one go and can avoid being added to deferred spli= t >> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still= 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., migrat= ion. >> >> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked >> 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 exclude >> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does no= t >> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still= >> 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_rmap= (struct folio *folio, >> * page of the folio is unmapped and at least one page= >> * is still mapped. >> */ >> - if (folio_test_large(folio) && folio_test_anon(folio))= >> - if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_pmd= mapped) >> - 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(mappe= d)) || >> + (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapped= ))) >> + deferred_split_folio(folio); > > Hi Zi Yan, > in case a mTHP is mapped by two processed (forked but not CoW yet), if = we > unmap the whole folio by pte level in one process only, are we still ad= ding this > folio into deferred list? No. Because the mTHP is still fully mapped by the other process. In terms= of code, nr will be 0 in that case and this if condition is skipped. nr is only in= creased from 0 when one of the subpages in the mTHP has no mapping, namely page->= _mapcount becomes negative and last is true in the case RMAP_LEVEL_PTE. -- Best Regards, Yan, Zi --=_MailMate_5B2280AB-7499-4B5A-95DA-0149144A2252_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmYrCakPHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhUGDQP/3KsulRt1LZDlngj5Ml/Zs9sN8F2IyJrBhMe px7IDX9JJ/8M5PrKwzo18xfTjvWLyf+9TLtf0+rFtSH2ewsW1JcigXsul/T+Del5 wXeybrKPvbfQOIf6uMZtgAgsB3abhbGeusqjCUryQVM+HQIwLq1tJ0ukHnLdijAZ mFgPb4OAi8T5ocuiC0jOPH//5xkAbat9Wb7Wt+ovjNkGlAssmp3BUSFIWPCTWEYO swZc4GKyWhB2xz6gOCF6qdqmrGPcKA+XEVwCXgMX4zPAZaIK5ERv2KWJ8qaj3ggh ztap+jayjEbwmIAeOo0yg/v218ue0x5sRbAqHKwD1vujwpuOvdtBRPsowowSTrJm mMkNtTQP8uULE4HlzB0T4F5RnSck2iK8pUF8VKT0JijP2EQ/QQ8p0uoffNrIdSiF IuwgfAwFtOz40BzzMQR67EFLkOvKgJ2ykStl/Ix6NTCYHeeAlEKxh+KkfeLlJcRo RpUz1qC7WgYryu+QknDkIYzgdij7mtXl9SyCSlB3x/MGoJ04COLZx3TeHBIp2F53 1/vHUedx3HpIh8uRleFlX4+198lqZuNNLHCP+KnGvXfqnwKeAOQUqTHs8lZVpZ+S jK2kf7/hj0tWtLRTHzA1lVrovawmRAVyehq9Qa9102LtYtQWe2OVnXZzTYvlUkHT V1NtIdfS =lI1h -----END PGP SIGNATURE----- --=_MailMate_5B2280AB-7499-4B5A-95DA-0149144A2252_=--