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([2620:10d:c092:500::4:4f66]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45dd4affb6bsm51876115e9.1.2025.09.05.08.01.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Sep 2025 08:01:27 -0700 (PDT) Message-ID: <8461f6df-a958-4c34-9429-d6696848a145@gmail.com> Date: Fri, 5 Sep 2025 16:01:23 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default Content-Language: en-GB To: David Hildenbrand , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Andrew Morton , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song References: <20250905141137.3529867-1-david@redhat.com> <06874db5-80f2-41a0-98f1-35177f758670@gmail.com> <1aa5818f-eb75-4aee-a866-9d2f81111056@redhat.com> <8b9ee2fe-91ef-4475-905c-cf0943ada720@gmail.com> From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 05/09/2025 15:58, David Hildenbrand wrote: > On 05.09.25 16:53, Usama Arif wrote: >> >> >> On 05/09/2025 15:46, David Hildenbrand wrote: >>> [...] >>> >>>> >>>> The reason I did this is for the case if you change max_ptes_none after the THP is added >>>> to deferred split list but *before* memory pressure, i.e. before the shrinker runs, >>>> so that its considered for splitting. >>> >>> Yeah, I was assuming that was the reason why the shrinker is enabled as default. >>> >>> But in any sane system, the admin would enable the shrinker early. If not, we can look into handling it differently. >> >> Yes, I do this as well, i.e. have a low value from the start. >> >> Does it make sense to disable shrinker if max_ptes_none is 511? It wont shrink >> the usecase you are describing below, but we wont encounter the increased CPU usage.> > > I don't really see why we should do that. > > If the shrinker is a problem than the shrinker should be disabled. But if it is enabled, we should be shrinking as documented. > > Without more magic around our THP toggles (we want less) :) > > Shrinking happens when we are under memory pressure, so I am not really sure how relevant the scanning bit is, and if it is relevant enought to change the shrinker default. > yes agreed, I also dont have numbers to back up my worry, its all theoretical :)