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From: Lance Yang In-Reply-To: <70da1948-b0f2-f54b-e0eb-ff901e007ccc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 55D4BC0004 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: 9etxjyrmen8naw7hzmi6qjioszcpy4gc X-HE-Tag: 1758006353-527120 X-HE-Meta: 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 Bpqi4tmL uhpPSI1yAAHwqP8USj5Ui5RqJcEB/w+b7weNboIl6RckS2dDQrVkxz+Yiv4JobXU3/hMr9wwcc4X78nbphx5Tn7re8JKrgbJcZWNAjirIotxEaaTVuMUgBEjl3GPHmseSHhmtIh0tFS11jfWFi+q11icAbVUuqbzu+ig9gQ+cfR3bYtHmNniJfjayXHwUAQ3WRbujfRLJK/iXxbHh1E6fQ8bad3k9/btNfF1fww18ImdK5DGt0YeqdYBqLiVJJCOpIGFt 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: On 2025/9/16 14:42, Hugh Dickins wrote: > On Tue, 16 Sep 2025, Lance Yang wrote: > >> Hi Hugh, >> >> Thanks for taking a look and for raising this important point! >> >> On 2025/9/16 13:32, Hugh Dickins wrote: >>> On Sun, 14 Sep 2025, Lance Yang wrote: >>> >>>> From: Lance Yang >>>> >>>> Let's skip unsuitable VMAs early in the khugepaged scan; specifically, >>>> mlocked VMAs should not be touched. >>> >>> Why? That's a change in behaviour, isn't it? >>> >>> I'm aware that hugepage collapse on an mlocked VMA can insert a fault >>> latency, not universally welcome; but I've not seen discussion, let >>> alone agreement, that current behaviour should be changed. >>> Somewhere in yet-to-be-read mail? Please give us a link. >>> >>> Hugh >> >> You're right, this is indeed a change in behaviour. But it's specifically >> for khugepaged. >> >> Users of mlock() expect low and predictable latency. THP collapse is a >> heavy operation that introduces exactly the kind of unpredictable delays >> they want to avoid. It has to unmap PTEs, copy data from the small folios >> to a new THP, and then remap the THP back to the PMD ;) >> >> IMO, that change is acceptable because THP is generally transparent to >> users, and khugepaged does not guarantee when THP collapse or split will >> happen. > > I disagree. Many of those who have khugepaged enabled would prefer > it to give them hugepages, even or especially on mlocked areas. > > If you make that change, it must be guarded by a sysfs or sysctl tuning. Thanks for the feedback! Well, seems like we're not on the same page. Let's gather more opinions from other folks ;) > > Perhaps it could share the sysctl_compact_unevictable_allowed tuning > (I'm not sure whether that's a good or bad idea: opinions will differ). Thanks, Lance > > Hugh > >> >> Well, we don't have a discussion on that, just something I noticed. >> >> Thanks, >> Lance