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[2003:d8:2f25:700:d846:15f3:6ca0:8029]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4b48f7a531asm102949671cf.49.2025.09.08.05.04.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 05:04:08 -0700 (PDT) Message-ID: Date: Mon, 8 Sep 2025 14:04:05 +0200 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 v2 1/1] mm: skip mlocked THPs that are underused early in deferred_split_scan() To: Kiryl Shutsemau Cc: Lance Yang , akpm@linux-foundation.org, Liam.Howlett@oracle.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, npache@redhat.com, ryan.roberts@arm.com, usamaarif642@gmail.com, ziy@nvidia.com References: <20250908090741.61519-1-lance.yang@linux.dev> <9a0c07b9-5bf0-4251-8609-fbaf0ca75bf9@redhat.com> <5j6i2o6umqwxabdfncbrdytmvdma4yrraxe6hu4csckcniduya@sm3mlablwbad> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 08.09.25 13:44, Kiryl Shutsemau wrote: > On Mon, Sep 08, 2025 at 01:32:05PM +0200, David Hildenbrand wrote: >> On 08.09.25 12:38, Kiryl Shutsemau wrote: >>> On Mon, Sep 08, 2025 at 05:07:41PM +0800, Lance Yang wrote: >>>> From: Lance Yang >>>> >>>> When we stumble over a fully-mapped mlocked THP in the deferred shrinker, >>>> it does not make sense to try to detect whether it is underused, because >>>> try_to_map_unused_to_zeropage(), called while splitting the folio, will not >>>> actually replace any zeroed pages by the shared zeropage. >>> >>> It makes me think, does KSM follows the same logic as >>> try_to_map_unused_to_zeropage()? >>> >>> I cannot immediately find what prevents KSM from replacing zeroed mlocked >>> folio with ZERO_PAGE(). >>> >>> Hm? >> >> I assume if you're using mlock and at the same time enable KSM for a >> process/VMA, you're doing something wrong. >> >> In contrast, THP is supposed to be transparent (yeah, I know ...). > > Yeah, I guess it is user error. > > Maybe we should make ksm_compatible() return false for VM_LOCKED? > KSM breaks mlock() contract. I was thinking the same and falsely remembered that we would already be checking for that. > > But it can be risky if someone already relies on this broken behaviour. Could be. Staring at QEMU, we have the following parameters: mem-merge=on|off Enables or disables memory merge support. This feature, when supported by the host, de-duplicates identical memory pages among VMs instances (enabled by default). And -overcommit mem-lock=on|off|on-fault "Run qemu with hints about host resource overcommit. The default is to assume that host overcommits all resources." Now, I would assume that anybody who sets "-overcommit mem-lock=on" either (a) Has KSM disabled on that machine. (b) Sets mem-merge=off as well. But QEMU would allow for configuring it. Interestingly, mm_populate()->populate_vma_page_range() wants to break COW. [*] But if the app later calls fork(), we still allow for cow-sharing pages with the child. (another case of "don't do it", like KSM I guess) [*] it doesn't do it for mappings that start out R/O. I think we might end up with sharedzero pages in that case, but not sure if worth fixing. -- Cheers David / dhildenb