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([2620:10d:c092:500::4:4f66]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3cf33add504sm31413426f8f.30.2025.09.05.08.02.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Sep 2025 08:02:11 -0700 (PDT) Message-ID: <74906d5a-a838-4f0a-8f07-532c787d3540@gmail.com> Date: Fri, 5 Sep 2025 16:02:08 +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> From: Usama Arif In-Reply-To: <20250905141137.3529867-1-david@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 05/09/2025 15:11, David Hildenbrand wrote: > We added an early exit in thp_underused(), probably to avoid scanning > pages when there is no chance for success. > > However, assume we have max_ptes_none = 511 (default). > > Nothing should stop us from freeing all pages part of a THP that > is completely zero (512) and khugepaged will for sure not try to > instantiate a THP in that case (512 shared zeropages). > > This can just trivially happen if someone writes a single 0 byte into a > PMD area, or of course, when data ends up being zero later. > > So let's remove that early exit. > > Do we want to CC stable? Hm, not sure. Probably not urgent. > > Note that, as default, the THP shrinker is active > (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all > THPs are added to the deferred split lists. However, with the > max_ptes_none default we would never scan them. We would not do that. If > that's not desirable, we should just disable the shrinker as default, > also not adding all THPs to the deferred split lists. > > Easy to reproduce: > > 1) Allocate some THPs filled with 0s > > > #include > #include > #include > #include > #include > > const size_t size = 1024*1024*1024; > > int main(void) > { > size_t offs; > char *area; > > area = mmap(0, size, PROT_READ | PROT_WRITE, > MAP_ANON | MAP_PRIVATE, -1, 0); > if (area == MAP_FAILED) { > printf("mmap failed\n"); > exit(-1); > } > madvise(area, size, MADV_HUGEPAGE); > > for (offs = 0; offs < size; offs += getpagesize()) > area[offs] = 0; > pause(); > } > <\prog.c> > > 2) Trigger the shrinker > > E.g., memory pressure through memhog > > 3) Observe that THPs are not getting reclaimed > > $ cat /proc/`pgrep prog`/smaps_rollup > > Would list ~1GiB of AnonHugePages. With this fix, they would get > reclaimed as expected. > > Fixes: dafff3f4c850 ("mm: split underused THPs") > Cc: Andrew Morton > Cc: Lorenzo Stoakes > Cc: Zi Yan > Cc: Baolin Wang > Cc: "Liam R. Howlett" > Cc: Nico Pache > Cc: Ryan Roberts > Cc: Dev Jain > Cc: Barry Song > Cc: Usama Arif > Signed-off-by: David Hildenbrand Acked-by: Usama Arif