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

[David Hildenbrand (Arm)]

On 3/13/26 20:59, Usama Arif wrote:
> 
> 
> 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?

I might be wrong, but I think you are mixing two things here:

(1) "Minimum" folio size (exec_folio_order())

(2) VMA alignment.


(2) should certainly be as large as (1), but assume we can get a 2M
folio on arm64 4k, why shouldn't we align it to 2M if the region is
reasonably sized, and use a PMD?


-- 
Cheers,

David