Re: [PATCH v4 3/4] mm: Optimize mprotect() by PTE-batching

[Dev Jain <dev.jain@arm.com>]

On 28/06/25 5:04 pm, Dev Jain wrote:
> Use folio_pte_batch to batch process a large folio. Reuse the folio from
> prot_numa case if possible.
>
> For all cases other than the PageAnonExclusive case, if the case holds true
> for one pte in the batch, one can confirm that that case will hold true for
> other ptes in the batch too; for pte_needs_soft_dirty_wp(), we do not pass
> FPB_IGNORE_SOFT_DIRTY. modify_prot_start_ptes() collects the dirty
> and access bits across the batch, therefore batching across
> pte_dirty(): this is correct since the dirty bit on the PTE really is
> just an indication that the folio got written to, so even if the PTE is
> not actually dirty (but one of the PTEs in the batch is), the wp-fault
> optimization can be made.
>
> The crux now is how to batch around the PageAnonExclusive case; we must
> check the corresponding condition for every single page. Therefore, from
> the large folio batch, we process sub batches of ptes mapping pages with
> the same PageAnonExclusive condition, and process that sub batch, then
> determine and process the next sub batch, and so on. Note that this does
> not cause any extra overhead; if suppose the size of the folio batch
> is 512, then the sub batch processing in total will take 512 iterations,
> which is the same as what we would have done before.
>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>   

Forgot to add:

Co-developed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

as this patch is almost identical to the diff Ryan had suggested.