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([2a02:6b6f:e750:f900:146f:2c4f:d96e:4241]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b4cc7bsm2486501f8f.86.2025.06.13.07.23.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 13 Jun 2025 07:23:20 -0700 (PDT) Message-ID: Date: Fri, 13 Jun 2025 15:23:19 +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 v2 0/2] fix MADV_COLLAPSE issue if THP settings are disabled To: Baolin Wang , akpm@linux-foundation.org, hughd@google.com, david@redhat.com Cc: lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 05/06/2025 09:00, Baolin Wang wrote: > As we discussed in the previous thread [1], the MADV_COLLAPSE will ignore > the system-wide anon/shmem THP sysfs settings, which means that even though > we have disabled the anon/shmem THP configuration, MADV_COLLAPSE will still > attempt to collapse into a anon/shmem THP. This violates the rule we have > agreed upon: never means never. This patch set will address this issue. Hi Baolin, I know never means never, but I also thought that the per-size toggles had priority over the system ones. This was discussed in [1] as well. My understanding with these patches is that if we have: [root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/enabled always madvise [never] [root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled always inherit [madvise] never Than without these patches we get a hugepage when we do MADV_HUGEPAGE, but with these we won't get a hugepage anymore eventhough hugepages-2048kB/enabled is set to madvise? I know this isn't ABI, but this would break existing expectations. (For e.g. we have certain 64K page size arm machines with global enabled = never and 2M = madvise, and we want 2M hugepages to fault at madvise). If the whole thing was being implemented from scratch, we should have definitely done it this way, but this can give a people a nasty surprise when they upgrade the kernel and suddenly stop getting hugepages. [1] https://lore.kernel.org/all/97702ff0-fc50-4779-bfa8-83dc42352db1@redhat.com/ Thanks, Usama