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

[Kiryl Shutsemau]

On Fri, Mar 20, 2026 at 06:58:53AM -0700, Usama Arif wrote:
> For PIE binaries (ET_DYN), the load address is randomized at PAGE_SIZE
> granularity via arch_mmap_rnd(). On arm64 with 64K base pages, this
> means the binary is 64K-aligned, but contpte mapping requires 2M
> (CONT_PTE_SIZE) alignment.
> 
> Without proper virtual address alignment, readahead patches that
> allocate 2M folios with 2M-aligned file offsets and physical addresses
> cannot benefit from contpte mapping, as the contpte fold check in
> contpte_set_ptes() requires the virtual address to be CONT_PTE_SIZE-
> aligned.
> 
> Fix this by extending maximum_alignment() to consider the maximum folio
> size supported by the page cache (via mapping_max_folio_size()). For
> each PT_LOAD segment, the alignment is bumped to the largest
> power-of-2 that fits within the segment size, capped by the max folio
> size the filesystem will allocate, if:
> 
>   - Both p_vaddr and p_offset are aligned to that size
>   - The segment is large enough (p_filesz >= size)
> 
> This ensures load_bias is folio-aligned so that file-offset-aligned
> folios map to properly aligned virtual addresses, enabling hardware PTE
> coalescing (e.g. arm64 contpte) and PMD mappings for large folios.
> 
> The segment size check avoids reducing ASLR entropy for small binaries
> that cannot benefit from large folio alignment.
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov