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List-Subscribe: List-Unsubscribe: On Fri, Sep 12, 2025 at 05:58:11PM +0100, 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. > > Signed-off-by: Kiryl Shutsemau LGTM, nice! Reviewed-by: Lorenzo Stoakes > --- > mm/khugepaged.c | 25 ++++++++++++++++++++++++- > 1 file changed, 24 insertions(+), 1 deletion(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index b486c1d19b2d..9e76a4f46df9 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1488,6 +1488,28 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, > return SCAN_SUCCEED; > } > > +static int install_huge_pmd(struct vm_area_struct *vma, unsigned long haddr, > + pmd_t *pmd, struct folio *folio) > +{ > + struct mm_struct *mm = vma->vm_mm; > + pgd_t *pgd; > + p4d_t *p4d; > + pud_t *pud; > + > + pgd = pgd_offset(mm, haddr); > + p4d = p4d_alloc(mm, pgd, haddr); > + if (!p4d) > + return SCAN_FAIL; > + pud = pud_alloc(mm, p4d, haddr); > + if (!pud) > + return SCAN_FAIL; > + pmd = pmd_alloc(mm, pud, haddr); > + if (!pmd) > + return SCAN_FAIL; > + > + return set_huge_pmd(vma, haddr, pmd, folio, &folio->page); > +} > + > /** > * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at > * address haddr. > @@ -1556,6 +1578,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > switch (result) { > case SCAN_SUCCEED: > break; > + case SCAN_PMD_NULL: > case SCAN_PMD_NONE: > /* > * All pte entries have been removed and pmd cleared. > @@ -1700,7 +1723,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > maybe_install_pmd: > /* step 5: install pmd entry */ > result = install_pmd > - ? set_huge_pmd(vma, haddr, pmd, folio, &folio->page) > + ? install_huge_pmd(vma, haddr, pmd, folio) > : SCAN_SUCCEED; > goto drop_folio; > abort: > -- > 2.50.1