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

Matthew Wilcox
2026-03-30T12:56:22Z

On Sun, Mar 29, 2026 at 12:37:00PM +0800, WANG Rui wrote:
> > mapping_max_folio_size() reflects what the page cache will actually
> > allocate for a given filesystem, since readahead caps folio allocation
> > at mapping_max_folio_order() (in page_cache_ra_order()). If btrfs
> > reports PAGE_SIZE, readahead won't allocate large folios for it, so
> > there are no large folios to coalesce PTEs for, aligning the binary
> > beyond that would only reduce ASLR entropy for no benefit.
> >
> > I don't think we should over-align binaries on filesystems that can't
> > take advantage of it.
>
> Ah, it looks like this might be overlooking another path that can create
> huge page mappings for read-only code segments: even when the filesystem
> (e.g. btrfs without experimental) didn't support large folios,
> READ_ONLY_THP_FOR_FS still allowed read-only file-backed code segments
> to be collapsed into huge page mappings via khugepaged.
>
> As Wilcox pointed out, it may take quite some time for many filesystems
> to gain full large folio support? So what I'm trying to clarify is that
> using mapping_max_folio_size() on this path is not favorable for
> khugepaged-based optimizations.

Nono, that's not what I'm pointing out! btrfs is simply not putting
in the effort to support large folios, and that needs to change.
READ_ONLY_THP_FOR_FS unnecessaily burdens the rest of the kernel.
It was a great hack for its time and paved the path for a lot of what
we have today, but it's time to remove it.