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

On 20/03/2026 18:58, Matthew Wilcox wrote:

On Fri, Mar 20, 2026 at 06:58:53AM -0700, Usama Arif wrote:

quoted

-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?

Yeah its unnecessary here. Its used in other places and this is never
checked, so we can remove it.

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

ack. will change in next revision.

quoted

+
+			/*
+			 * 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;

ack, will try and address this accordingly.


Thanks for the reviews!!