Re: [PATCH 1/4] arm64: request contpte-sized folios for exec memory

[David Hildenbrand (Arm)]

On 3/10/26 15:51, Usama Arif wrote:
> exec_folio_order() was introduced [1] to request readahead of executable
> file-backed pages at an arch-preferred folio order, so that the hardware
> can coalesce contiguous PTEs into fewer iTLB entries (contpte).
> 
> The current implementation uses ilog2(SZ_64K >> PAGE_SHIFT), which
> requests 64K folios. This is optimal for 4K base pages (where CONT_PTES
> = 16, contpte size = 64K), but suboptimal for 16K and 64K base pages:
> 
> Page size | Before (order) | After (order) | contpte
> ----------|----------------|---------------|--------
> 4K        | 4 (64K)        | 4 (64K)       | Yes (unchanged)
> 16K       | 2 (64K)        | 7 (2M)        | Yes (new)
> 64K       | 0 (64K)        | 5 (2M)        | Yes (new)
> 
> For 16K pages, CONT_PTES = 128 and the contpte size is 2M (order 7).
> For 64K pages, CONT_PTES = 32 and the contpte size is 2M (order 5).
> 
> Use ilog2(CONT_PTES) instead, which directly evaluates to contpte-aligned
> order for all page sizes.
> 
> The worst-case waste is bounded to one folio (up to 2MB - 64KB)
> at the end of the file, since page_cache_ra_order() reduces the folio
> order near EOF to avoid allocating past i_size.

So, if you have a smallish text segment in a larger file, we'd always
try to allocate 2M on 16k/64k?

That feels wrong.

Asking the other way around: why not also use 2M on a 4k system and end
up with a PMD?

And no, I don't think we should default to that, just emphasizing my
point that *maybe* we really want to consider mapping (vma) size as well.

-- 
Cheers,

David