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

[David Hildenbrand (Arm)]

On 3/18/26 11:41, Usama Arif wrote:
> 
> 
> On 16/03/2026 19:06, David Hildenbrand (Arm) wrote:
>> On 3/13/26 20:59, Usama Arif wrote:
>>>
>>>
>>>
>>> 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?
>>
>>
> 
> So this series is tackling both (1) and (2). When I started making changes
> to the code, what I wanted was 2M folios at fault with 64K base page size
> to reduce iTLB misses. This is what patch 1 (and 2) will achieve.
> 
> Yes, completely agree, (2) should be as large as (1). I didn't think about
> PMD size on 4K which you pointed out. do_sync_mmap_readahead can give
> that with force_thp_readahead, so this should be supported.

In particular, imagine if hw starts optimizing transparently on other
granularity, then the "smallest granularity" (exec_folio_order())
decision will soon be wrong.

> 
> But we shouldn't align to PMD size for all base page sizes. As Rui pointed
> out, increasing alignment size reduces ASLR entropy [1]. Should we max alignement
> to 2M?

That's why I said that likely, as an input, we'd want to use the mapping
size or other heuristics.

We wouldn't want to align a 4k mapping to either 64k or 2M.

Long story short: the change in thp_get_unmapped_area_vmflags() needs
some thought IMHO.

-- 
Cheers,

David