Re: [PATCH v2 4/4] mm: align file-backed mmap to max folio order in thp_get_unmapped_area

Kiryl Shutsemau
2026-03-20T15:06:11Z

On Fri, Mar 20, 2026 at 06:58:54AM -0700, Usama Arif wrote:
> thp_get_unmapped_area() is the get_unmapped_area callback for
> filesystems like ext4, xfs, and btrfs. It attempts to align the virtual
> address for PMD_SIZE THP mappings, but on arm64 with 64K base pages
> PMD_SIZE is 512M, which is too large for typical shared library mappings,
> so the alignment always fails and falls back to PAGE_SIZE.
>
> This means shared libraries loaded by ld.so via mmap() get 64K-aligned
> virtual addresses, preventing contpte mapping even when 2M folios are
> allocated with properly aligned file offsets and physical addresses.
>
> Add a fallback in thp_get_unmapped_area_vmflags() that uses the
> filesystem's mapping_max_folio_size() to determine alignment, capped to
> the mapping length via rounddown_pow_of_two(len). This aligns mappings
> to the largest folio the page cache will actually allocate, without
> over-aligning small mappings.
>
> The fallback is naturally a no-op for filesystems that don't support
> large folios and skips the retry when the alignment would equal PMD_SIZE
> (already attempted above).
>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
> mm/huge_memory.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8e2746ea74adf..4005084c9c65b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1242,6 +1242,20 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
> if (ret)
> return ret;
>
> + if (filp && filp->f_mapping) {
> + unsigned long max_folio_size =
> + mapping_max_folio_size(filp->f_mapping);
> + unsigned long size = rounddown_pow_of_two(len);
> +
> + size = min(size, max_folio_size);
> + if (size > PAGE_SIZE && size != PMD_SIZE) {
> + ret = __thp_get_unmapped_area(filp, addr, len, off,
> + flags, size, vm_flags);

Have you considered to integrate inside __thp_get_unmapped_area?

Like, start with PMD_SIZE alignment, then lower to
mapping_max_folio_size if needed and then lower further based on mapping
size.

--
Kiryl Shutsemau / Kirill A. Shutemov