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 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 2025-02-28 16:45, David Hildenbrand wrote: > On 28.02.25 22:38, Mathieu Desnoyers wrote: >> On 2025-02-28 10:10, David Hildenbrand wrote: >> [...] >>> For example, QEMU will mark all guest memory is mergeable using MADV, to >>> limit the deduplicaton to guest RAM only. >>> >> >> On a related note, I think the madvise(2) documentation is inaccurate. >> >> It states: >> >>          MADV_MERGEABLE (since Linux 2.6.32) >>                 Enable  Kernel Samepage Merging (KSM) for the pages in >> the range >>                 specified by addr and length. [...] >> >> AFAIU, based on code review of ksm_madvise(), this is not strictly true. >> >> The KSM implementation enables KSM for pages in the entire vma >> containing the range. >> So if it so happens that two mmap areas with identical protection >> flags are merged, >> both will be considered mergeable by KSM as soon as at least one page >> from any of >> those areas is made mergeable. > > I *think* it does what is documented. In madvise_vma_behavior(), > ksm_madvise() will update "new_flags". > > Then we call madvise_update_vma() to split the VMA if required and set > new_flags only on the split VMA. The handling is similar to other MADV > operations that end up modifying vm_flags. > > If I am missing something and this is indeed broken, we should > definitely write a selftest for it and fix it. > You are correct, I missed that part. Thanks for the clarification! Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com