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

[WANG Rui]

> 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.

Thanks,
Rui