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

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/ (local)

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