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

Usama Arif
2026-03-30T14:01:12Z

On 30/03/2026 15:56, Matthew Wilcox wrote:
> 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.

ah yes, Thank you for pointing this out!

Maybe we should rename mapping_max_folio_size() to mapping_fault_max_folio_size().

>>
>> 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.

ack

I am worried that 32M is too large and we lose out on a lot of ASLR bits.
Instead of PMD_ORDER, should we do max(SZ_2M, PMD_ORDER)?

> 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.