Re: [PATCH 0/4] arm64/mm: contpte-sized exec folios for 16K and 64K pages

[Usama Arif <usama.arif@linux.dev>]

On Fri, 13 Mar 2026 13:55:38 -0700 Usama Arif <usama.arif@linux.dev> wrote:

> On Fri, 13 Mar 2026 16:33:42 +0000 Ryan Roberts <ryan.roberts@arm.com> wrote:
> 
> > On 10/03/2026 14:51, Usama Arif wrote:
> > > On arm64, the contpte hardware feature coalesces multiple contiguous PTEs
> > > into a single iTLB entry, reducing iTLB pressure for large executable
> > > mappings.
> > > 
> > > exec_folio_order() was introduced [1] to request readahead at an
> > > arch-preferred folio order for executable memory, enabling contpte
> > > mapping on the fault path.
> > > 
> > > However, several things prevent this from working optimally on 16K and
> > > 64K page configurations:
> > > 
> > > 1. exec_folio_order() returns ilog2(SZ_64K >> PAGE_SHIFT), which only
> > >    produces the optimal contpte order for 4K pages. For 16K pages it
> > >    returns order 2 (64K) instead of order 7 (2M), and for 64K pages it
> > >    returns order 0 (64K) instead of order 5 (2M). 
> > 
> > This was deliberate, although perhaps a bit conservative. I was concerned about
> > the possibility of read amplification; pointlessly reading in a load of memory
> > that never actually gets used. And that is independent of page size.
> > 
> > 2M seems quite big as a default IMHO, I could imagine Android might complain
> > about memory pressure in their 16K config, for example.
> > 
> 
> The force_thp_readahead path in do_sync_mmap_readahead() reads at
> HPAGE_PMD_ORDER (2M on x86) and even doubles it to 4M for
> non VM_RAND_READ mappings (ra->size *= 2), with async readahead
> enabled. exec_folio_order() is more conservative. a single 2M folio
> with async_size=0, no speculative prefetch. So I think the memory
> pressure would not be worse than what x86 has?
> 
> For memory pressure on Android 16K: the readahead is clamped to VMA
> boundaries, so a small shared library won't read 2M.
> page_cache_ra_order() reduces folio order near EOF and on allocation
> failure, so the 2M order is a preference, not a guarantee with the
> current code?
> 

I am not a big fan of introducing Kconfig options, but would
CONFIG_EXEC_FOLIO_ORDER with the default value being 64K be a better
solution? Or maybe a default of 64K for 4K and 16K base page size,
but 2M for 64K page size as 64K base page size is mostly for servers.

Using a default value of 64K would mean no change in behaviour.