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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 15 Sep 2025, at 9:52, Kiryl Shutsemau wrote: > From: Kiryl Shutsemau > > MADV_COLLAPSE on a file mapping behaves inconsistently depending on if > PMD page table is installed or not. > > Consider following example: > > p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE, > MAP_SHARED, fd, 0); > err = madvise(p, 2UL << 20, MADV_COLLAPSE); > > fd is a populated tmpfs file. > > The result depends on the address that the kernel returns on mmap(). > If it is located in an existing PMD table, the madvise() will succeed. > However, if the table does not exist, it will fail with -EINVAL. > > This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when > a page table is missing, which causes collapse_pte_mapped_thp() to fail. > > SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in > collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page > tables as needed. Why does collapse code want to know the difference between SCAN_PMD_NULL and SCAN_PMD_NONE? Both seems to be treated as “nothing here, install a PMD leaf”. One difference is that madvise_collapse() will continue on SCAN_PMD_NULL but bail out on SCAN_PMD_NONE. I wonder if we could have SCAN_PMD_NULL_OR_NONE instead. Zach, since you added both, can you share some insight? Thanks. > > Signed-off-by: Kiryl Shutsemau > --- > > v2: > - Modify set_huge_pmd() instead of introducing install_huge_pmd(); > > --- > mm/khugepaged.c | 20 +++++++++++++++++++- > 1 file changed, 19 insertions(+), 1 deletion(-) > The changes look good to me. Reviewed-by: Zi Yan Best Regards, Yan, Zi