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

[Usama Arif]

On 13/03/2026 16:20, David Hildenbrand (Arm) wrote:
> On 3/10/26 15:51, 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). Patch 1 fixes this by
>>    using ilog2(CONT_PTES) which evaluates to the optimal order for all
>>    page sizes.
>>
>> 2. Even with the optimal order, the mmap_miss heuristic in
>>    do_sync_mmap_readahead() silently disables exec readahead after 100
>>    page faults. The mmap_miss counter tracks whether readahead is useful
>>    for mmap'd file access:
>>
>>    - Incremented by 1 in do_sync_mmap_readahead() on every page cache
>>      miss (page needed IO).
>>
>>    - Decremented by N in filemap_map_pages() for N pages successfully
>>      mapped via fault-around (pages found in cache without faulting,
>>      evidence that readahead was useful). Only non-workingset pages
>>      count and recently evicted and re-read pages don't count as hits.
>>
>>    - Decremented by 1 in do_async_mmap_readahead() when a PG_readahead
>>      marker page is found (indicates sequential consumption of readahead
>>      pages).
>>
>>    When mmap_miss exceeds MMAP_LOTSAMISS (100), all readahead is
>>    disabled. On 64K pages, both decrement paths are inactive:
>>
>>    - filemap_map_pages() is never called because fault_around_pages
>>      (65536 >> PAGE_SHIFT = 1) disables should_fault_around(), which
>>      requires fault_around_pages > 1. With only 1 page in the
>>      fault-around window, there is nothing "around" to map.
>>
>>    - do_async_mmap_readahead() never fires for exec mappings because
>>      exec readahead sets async_size = 0, so no PG_readahead markers
>>      are placed.
>>
>>    With no decrements, mmap_miss monotonically increases past
>>    MMAP_LOTSAMISS after 100 faults, disabling exec readahead
>>    for the remainder of the mapping.
>>    Patch 2 fixes this by moving the VM_EXEC readahead block
>>    above the mmap_miss check, since exec readahead is targeted (one
>>    folio at the fault location, async_size=0) not speculative prefetch.
>>
>> 3. Even with correct folio order and readahead, contpte mapping requires
>>    the virtual address to be aligned to CONT_PTE_SIZE (2M on 64K pages).
>>    The readahead path aligns file offsets and the buddy allocator aligns
>>    physical memory, but the virtual address depends on the VMA start.
>>    For PIE binaries, ASLR randomizes the load address at PAGE_SIZE (64K)
>>    granularity, giving only a 1/32 chance of 2M alignment. When
>>    misaligned, contpte_set_ptes() never sets the contiguous PTE bit for
>>    any folio in the VMA, resulting in zero iTLB coalescing benefit.
>>
>>    Patch 3 fixes this for the main binary by bumping the ELF loader's
>>    alignment to PAGE_SIZE << exec_folio_order() for ET_DYN binaries.
>>
>>    Patch 4 fixes this for shared libraries by adding a contpte-size
>>    alignment fallback in thp_get_unmapped_area_vmflags(). The existing
>>    PMD_SIZE alignment (512M on 64K pages) is too large for typical shared
>>    libraries, so this smaller fallback (2M) succeeds where PMD fails.
>>
>> I created a benchmark that mmaps a large executable file and calls
>> RET-stub functions at PAGE_SIZE offsets across it. "Cold" measures
>> fault + readahead cost. "Random" first faults in all pages with a
>> sequential sweep (not measured), then measures time for calling random
>> offsets, isolating iTLB miss cost for scattered execution.
>>
>> The benchmark results on Neoverse V2 (Grace), arm64 with 64K base pages,
>> 512MB executable file on ext4, averaged over 3 runs:
>>
>>   Phase      | Baseline     | Patched      | Improvement
>>   -----------|--------------|--------------|------------------
>>   Cold fault | 83.4 ms      | 41.3 ms      | 50% faster
>>   Random     | 76.0 ms      | 58.3 ms      | 23% faster
> 
> I'm curious: is a single order really what we want?
> 
> I'd instead assume that we might want to make decisions based on the
> mapping size.
> 
> Assume you have a 128M mapping, wouldn't we want to use a different
> alignment than, say, for a 1M mapping, a 128K mapping or a 8k mapping?
> 

So I see 2 benefits from this. Page fault and iTLB coverage. IMHO page
faults are not that big of a deal? If the text section is hot, it wont
get flushed after faulting in. So the real benefit comes from improved
iTLB coverage.

For a 128M mapping, 2M alignment gives 64 contpte entries. Aligning
to something larger (say 128M) wouldn't give any additional TLB
coalescing, each 2M-aligned region independently qualifies for contpte.

Mappings smaller than 2M can't benefit from contpte regardless of
alignment, so falling back to PAGE_SIZE would be the optimal behaviour.
Adding intermediate sizes (e.g. 512K, 128K) wouldn't map to any
hardware boundary and adds complexity without TLB benefit?