Re: [PATCH v2 3/4] elf: align ET_DYN base to max folio size for PTE coalescing

Matthew Wilcox
2026-03-20T15:58:46Z

On Fri, Mar 20, 2026 at 06:58:53AM -0700, Usama Arif wrote:
> -static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr)
> +static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr,
> + struct file *filp)
> {
> unsigned long alignment = 0;
> + unsigned long max_folio_size = PAGE_SIZE;
> int i;
>
> + if (filp && filp->f_mapping)
> + max_folio_size = mapping_max_folio_size(filp->f_mapping);

Under what circumstances can bprm->file be NULL?

Also we tend to prefer the name "file" rather than "filp" for new code
(yes, there's a lot of old code out there).

> +
> + /*
> + * Try to align the binary to the largest folio
> + * size that the page cache supports, so the
> + * hardware can coalesce PTEs (e.g. arm64
> + * contpte) or use PMD mappings for large folios.
> + *
> + * Use the largest power-of-2 that fits within
> + * the segment size, capped by what the page
> + * cache will allocate. Only align when the
> + * segment's virtual address and file offset are
> + * already aligned to the folio size, as
> + * misalignment would prevent coalescing anyway.
> + *
> + * The segment size check avoids reducing ASLR
> + * entropy for small binaries that cannot
> + * benefit.
> + */
> + if (!cmds[i].p_filesz)
> + continue;
> + size = rounddown_pow_of_two(cmds[i].p_filesz);
> + size = min(size, max_folio_size);
> + if (size > PAGE_SIZE &&
> + IS_ALIGNED(cmds[i].p_vaddr, size) &&
> + IS_ALIGNED(cmds[i].p_offset, size))
> + alignment = max(alignment, size);

Can this not all be factored out into a different function? Also, I
think it was done a bit better here:
https://lore.kernel.org/linux-fsdevel/20260313005211.882831-1-r@hev.cc/

+ if (!IS_ALIGNED(cmd->p_vaddr | cmd->p_offset, PMD_SIZE))
+ return false;