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Shutemov" Subject: [PATCH RFC v2 1/2] mm: Don't skip swap entry even if zap_details specified Date: Mon, 15 Nov 2021 21:49:50 +0800 Message-Id: <20211115134951.85286-2-peterx@redhat.com> X-Mailer: git-send-email 2.32.0 In-Reply-To: <20211115134951.85286-1-peterx@redhat.com> References: <20211115134951.85286-1-peterx@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="US-ASCII" X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CCC6C509210B X-Stat-Signature: 7wmyptob6w7n6nhtfpiyne3x6asub5mh Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=V2+NoBUL; spf=none (imf01.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1636984198-90651 Content-Transfer-Encoding: quoted-printable 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: This check existed since the 1st git commit of Linux repository, but at t= hat time there's no page migration yet so I think it's okay. With page migration enabled, it should logically be possible that we zap = some shmem pages during migration. When that happens, IIUC the old code could= have the RSS counter accounted wrong on MM_SHMEMPAGES because we will zap the = ptes without decreasing the counters for the migrating entries. I have no uni= t test to prove it as I don't know an easy way to trigger this condition, though= . Besides, the optimization itself is already confusing IMHO to me in a few= points: - The wording "skip swap entries" is confusing, because we're not skipp= ing all swap entries - we handle device private/exclusive pages before that. - The skip behavior is enabled as long as zap_details pointer passed ov= er. It's very hard to figure that out for a new zap caller because it's u= nclear why we should skip swap entries when we have zap_details specified. - With modern systems, especially performance critical use cases, swap entries should be rare, so I doubt the usefulness of this optimizatio= n since it should be on a slow path anyway. - It is not aligned with what we do with huge pmd swap entries, where i= n zap_huge_pmd() we'll do the accounting unconditionally. This patch drops that trick, so we handle swap ptes coherently. Meanwhil= e we should do the same mapping check upon migration entries too. Signed-off-by: Peter Xu --- mm/memory.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 8f1de811a1dc..e454f3c6aeb9 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1382,16 +1382,14 @@ static unsigned long zap_pte_range(struct mmu_gat= her *tlb, continue; } =20 - /* If details->check_mapping, we leave swap entries. */ - if (unlikely(details)) - continue; - if (!non_swap_entry(entry)) rss[MM_SWAPENTS]--; else if (is_migration_entry(entry)) { struct page *page; =20 page =3D pfn_swap_entry_to_page(entry); + if (unlikely(zap_skip_check_mapping(details, page))) + continue; rss[mm_counter(page)]--; } if (unlikely(!free_swap_and_cache(entry))) --=20 2.32.0