Re: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD_NULL

[David Hildenbrand <david@redhat.com>]

On 15.09.25 15:52, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@kernel.org>
> 
> 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 <kas@kernel.org>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb