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Wed, 26 Aug 2026 04:43:55 +0000 X-Mizu-Trace-ID: d945f1d31c09ef98 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: pedrodemargomes@gmail.com Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH mm-stable] mm/khugepaged: avoid unnecessary checking for swap entries when collapsing a mTHP Date: Wed, 26 Aug 2026 12:43:50 +0800 Message-Id: <20260826044350.24284-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: <20260825192433.3185880-1-pedrodemargomes@gmail.com> References: <20260825192433.3185880-1-pedrodemargomes@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: 947wc96nownmiuwjpcnsa9o8hyxdd5tt X-Rspamd-Queue-Id: A9A95140006 X-HE-Tag: 1787719437-14427 X-HE-Meta: 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 JvOPj99L gG1xD48KZldkItlVhSNTTkBfvZjsVB2W2rlXTaRH1/vtDAKMdF3FaADwkaqFSUTRV9RDryrOiPUb+iZYAtT8TaMALNVBnpnmHUfvLhIFJZF4unX5DsdipJhwEXxB9pd3uUKZ52fnoOn+EEyzeVjArW7mlT//gOW8JlyjUAYZ7x/YtjViz71Qw/HA7P6tTcXJjM2Z11L5q4CdhwATM/UnkOQhl4/yQsDeaBIxMvXSvBvNdAY2g8wVFjLQQAcRuNRZZjrPIF8dZQ+uBuOrzNyNMnvQbWH8gZo7hN7R9Xon1ROmIUEjlxwkB4VbEtA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 25, 2026 at 04:24:33PM -0300, Pedro Demarchi Gomes wrote: >mthp_collapse() tries to swap in PTEs when collapsing a mTHP if there are any >swap PTEs in the PMD range, even if none of those swap PTEs are >actually part of the mTHP's range. > >Track swap PTEs in cc->mthp_unmapped_ptes so that mthp_collapse() can >tell whether the mTHP's own range contains a swap PTE, and skip the >swapin check when it doesn't. > >Signed-off-by: Pedro Demarchi Gomes >--- Yeah, the code itself is fine, but ... Any numbers showing how much the extra scan costs? khugepaged is already quite a mess and badly needs some cleanup. TBH, another bitmap + more bookkeeping doesn't feel worth it unless the win is pretty clear :) > mm/khugepaged.c | 29 +++++++++++++++++++---------- > 1 file changed, 19 insertions(+), 10 deletions(-) > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c >index 11ff98d55c76..38596172f7dc 100644 >--- a/mm/khugepaged.c >+++ b/mm/khugepaged.c >@@ -117,6 +117,9 @@ struct collapse_control { > > /* Each bit represents a single occupied (!none/zero) page. */ > DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE); >+ >+ /* Each bit represents a single not present and not none/zero pte. */ >+ DECLARE_BITMAP(mthp_unmapped_ptes, MAX_PTRS_PER_PTE); BTW, the comment is a bit of a mouthful :) How about "Each bit represents a non-present, non-none PTE." ? Cheers, Lance [...]