Re: [PATCH v2 2/2] mm/memory: reuse the whole exclusive large folio on a write fault

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/19/26 09:31, Yuan-Hao Hsu wrote:
> With blocks of 16 PTEs a PTE-mapped 2M THP still takes 32 reuse faults
> and a 1M folio 16.  Lift the bound to the page table: the walk then
> covers the PTEs that map the folio in this page table, at most
> PTRS_PER_PTE of them, and one fault does the work for the folio.
> 
> Its cost, measured as the time of the store that takes it, against
> 420 ns for a reuse fault today (x86-64, i7-12700KF, medians, ns):
> 
>                           reuse fault   fault that     COW fault that
>                           (patched)     allocated it   copies 4K
>       1M mTHP            5,400- 5,700   63,000-68,000          1,500
>       2M THP, PTE-mapped  10,000-10,500         126,000          1,500
> 
> That is 14-20 ns per PTE, about 2 ns of it the scan.  Builds that
> differ only by NOPs in front of the function take either 10,100 or
> 7,500 ns for the 2M folio, with a period of 32 bytes: the loop of
> modify_prot_commit_ptes() changes speed with its address.
> 
> What it buys, 256 MiB after fork() and the child's exit, medians of 15
> runs, two boots of each kernel:
> 
>                                    16 PTEs         whole folio
>   one byte per page, seq
>       1M mTHP                     5.7 /  5.8 ms    4.1 /  4.0 ms
>       2M THP, PTE-mapped          5.7 /  5.8 ms    3.9 /  3.9 ms
>   one byte per page, random order
>       2M THP, PTE-mapped          7.4 /  7.5 ms    4.9 /  5.1 ms
>   memset()
>       2M THP, PTE-mapped         40.2 / 39.4 ms   38.1 / 37.2 ms
>   8 threads, random order
>       2M THP, PTE-mapped          0.9 /  1.0 ms    0.7 /  0.7 ms
>   one store per 64K, 2M folios    3.3 /  3.3 ms    1.5 /  1.6 ms
>   one store per 2M, 2M folios     0.1 /  0.1 ms    1.4 /  1.4 ms
> 

Are you measuring the worst-case latency of a single write fault or only the
average over all writes?

-- 
Cheers,

David