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Received: from CAMSVWEXC02.scsc.local (2002:6a01:e348::6a01:e348) by CAMSVWEXC02.scsc.local (2002:6a01:e348::6a01:e348) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Tue, 15 Aug 2023 14:25:09 +0100 Received: from CAMSVWEXC02.scsc.local ([::1]) by CAMSVWEXC02.scsc.local ([fe80::3c08:6c51:fa0a:6384%13]) with mapi id 15.00.1497.012; Tue, 15 Aug 2023 14:25:09 +0100 From: Daniel Gomez To: Yin Fengwei , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "stable@vger.kernel.org" , "akpm@linux-foundation.org" , "willy@infradead.org" , "vishal.moola@gmail.com" , "wangkefeng.wang@huawei.com" , "minchan@kernel.org" , "yuzhao@google.com" , "david@redhat.com" , "ryan.roberts@arm.com" , "shy828301@gmail.com" Subject: Re: [PATCH v2 3/3] madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check Thread-Topic: [PATCH v2 3/3] madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check Thread-Index: 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 T95N5ZCv OCNqZRl4kRxxK/LljTpEvYRd37OtuAAjRQHjXh+A7yDisSt3yv2pEyIWQMlUP+8+XKMIMTxlDj/tOrqS+gIkqX4UM7G6BeDN7Qbg5SNIpQpyLigd0Whluht39C6G3M4t4zNwhC5mA26gy2W9av+3JzytfXfg+nQtDojMi1aaIzdEwRBAymZh+8iAI1zLa89N0EHqEixEpsqFTixHCNsXJ4vBhe0iFS1Bqx/vfPFoDsPxoXDCGJdM8RMlJ0bdO7iN/+k9T9tG8Bh4kpQZJQiYw9r3muQ== 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: Hi Yin, On Tue, Aug 08, 2023 at 10:09:17AM +0800, Yin Fengwei wrote: > Commit 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a > folio") replaced the page_mapcount() with folio_mapcount() to check > whether the folio is shared by other mapping. > > It's not correct for large folios. folio_mapcount() returns the total > mapcount of large folio which is not suitable to detect whether the folio > is shared. > > Use folio_estimated_sharers() which returns a estimated number of shares. > That means it's not 100% correct. It should be OK for madvise case here. I'm trying to understand why it should be ok for madvise this change, so I hope it's okay to ask you few questions. folio_mapcount() calculates the total maps for all the subpages of a folio. However, the folio_estimated_sharers does it only for the first subpage making it not true for large folios. Then, wouldn't this change drop support for large folios? Seems like folio_entire_mapcount() is not accurate either because of it does not inclue PTE-mapped sub-pages which I think we need here. Hence, the folio_mapcount(). Could this be something missing in the test side? I tried to replicate the setup with CONFIG_TRANSPARENT_HUGEPAGE but seems like I'm not able to do it: ./cow # [INFO] detected THP size: 2048 KiB # [INFO] detected hugetlb size: 2048 KiB # [INFO] detected hugetlb size: 1048576 KiB # [INFO] huge zeropage is enabled TAP version 13 1..166 # [INFO] Anonymous memory tests in private mappings # [RUN] Basic COW after fork() ... with base page not ok 1 MADV_NOHUGEPAGE failed # [RUN] Basic COW after fork() ... with swapped out base page not ok 2 MADV_NOHUGEPAGE failed # [RUN] Basic COW after fork() ... with THP not ok 3 MADV_HUGEPAGE failed # [RUN] Basic COW after fork() ... with swapped-out THP not ok 4 MADV_HUGEPAGE failed # [RUN] Basic COW after fork() ... with PTE-mapped THP not ok 5 MADV_HUGEPAGE failed # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP not ok 6 MADV_HUGEPAGE failed ... Daniel > > User-visible effects is that the THP is skipped when user call madvise. > But the correct behavior is THP should be split and processed then. > > NOTE: this change is a temporary fix to reduce the user-visible effects > before the long term fix from David is ready. > > Fixes: 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a = folio") > Cc: stable@vger.kernel.org > Signed-off-by: Yin Fengwei > Reviewed-by: Yu Zhao > Reviewed-by: Ryan Roberts > --- > mm/madvise.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/madvise.c b/mm/madvise.c > index 49af35e2d99a..4dded5d27e7e 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -683,7 +683,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigne= d long addr, > if (folio_test_large(folio)) { > int err; > > - if (folio_mapcount(folio) !=3D 1) > + if (folio_estimated_sharers(folio) !=3D 1) > break; > if (!folio_trylock(folio)) > break; > -- > 2.39.2 >=