Re: [PATCH RFC] mm/memory: drop PROCESS_PAGES_NON_PREEMPT_BATCH

[Pedro Falcato]

On Thu, Oct 22, 2026 at 08:14:30AM +0000, Shivank Garg wrote:
> clear_contig_highpages() clears the whole range at once under
> preemptible models. Otherwise it clears PROCESS_PAGES_NON_PREEMPT_BATCH
> pages at a time, with a cond_resched() in between. The batch is 32MB if
> the architecture defines clear_pages(), and one page otherwise.
> 
> Since commit 7dadeaa6e851 ("sched: Further restrict the preemption
> modes"), architectures that support lazy preemption only offer the
> full, lazy and RT preemption models, also with PREEMPT_DYNAMIC. x86 is
> one of them, and it is the only architecture that defines
> clear_pages(). So the 32MB batch is never used: whenever
> clear_contig_highpages() runs under a cooperative model, the batch is
> one page.
> 
> Make that explicit. Clear the whole range when CONFIG_PREEMPTION is
> set, and one page at a time with cond_resched() in between otherwise.
> Drop PROCESS_PAGES_NON_PREEMPT_BATCH, which has no other users.
> 
> PREEMPT_DYNAMIC kernels also have CONFIG_PREEMPTION set, and
> cond_resched() does nothing on any of these kernels. So a build-time
> CONFIG_PREEMPTION check is enough, and preempt_model_preemptible() is
> not needed.
> 
> No functional change intended.
> 
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
> ---
> This came up in review of the copy_mc_highpages() series [1]. Pedro
> pointed out that most architecture are full or lazy preempt, so mm does
> not need to check the preemption model at runtime or batch the copy for
> cond_resched(), and simple per-page fallback is enough for non-preemptible
> models. David suggested doing the same for the zeroing path, and
> this patch does that.
> 
> [1] https://lore.kernel.org/all/20261019-batpagecopy-v2-1-d75e7d0dfaae@xxxxxxx 
> ---
>  include/linux/mm.h | 28 +---------------------------
>  mm/memory.c        | 29 ++++++++++++++++-------------
>  2 files changed, 17 insertions(+), 40 deletions(-)

Awesome!

> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 35f5fc43861a..1083d0645d63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -5269,7 +5269,7 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
>   *
>   * When running under preemptible models this is not a problem. Under
>   * cooperatively scheduled models, however, the caller is expected to
> - * limit @npages to no more than PROCESS_PAGES_NON_PREEMPT_BATCH.
> + * clear one page at a time and call cond_resched() in between.

Why can't clear_pages() itself do cond_resched()? Same question for
clear_user_pages() and/or clear_user_highpages().

-- 
Pedro