Re: [PATCH v10 18/37] mm: page_alloc: use aliasing checks instead of user_alloc_needs_zeroing

[Lorenzo Stoakes]

On Mon, Jun 08, 2026 at 04:38:00AM -0400, Michael S. Tsirkin wrote:
> Replace user_alloc_needs_zeroing() with the direct aliasing checks
> (cpu_dcache_is_aliasing() || cpu_icache_is_aliasing()) in the
> post_alloc_hook aliasing guard.
>
> user_alloc_needs_zeroing() includes a !init_on_alloc term that
> means "allocator didn't zero this page."  But in this guard's
> context (!zeroed && !init && __GFP_ZERO), we already know the page
> is zero; init incorporates init_on_alloc via want_init_on_alloc().
> The only question left is whether the cache architecture needs
> the data re-zeroed through a congruent mapping, which is purely
> cpu_dcache_is_aliasing() || cpu_icache_is_aliasing().
>
> On non-aliasing architectures with init_on_free=true and
> init_on_alloc=false, this avoids a redundant re-zero of an
> already-zero page.
>
> Note on PowerPC: PowerPC overrides clear_user_page to call
> flush_dcache_page after clear_page, but on freshly allocated
> pages PG_dcache_clean is already clear (cleared by
> __free_pages_prepare), so flush_dcache_page is a no-op.
> Skipping this here thus has no effect.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-6

This seems like an odd ordering of patches, can we group like changes
together?

> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 45e824b1ec75..edfc83571985 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1880,7 +1880,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>  	 */
>  	if (!zeroed && !init && (gfp_flags & __GFP_ZERO) &&
>  	    user_addr != USER_ADDR_NONE &&
> -	    user_alloc_needs_zeroing())
> +	    (cpu_dcache_is_aliasing() || cpu_icache_is_aliasing()))

Let's try and simplify things rather than adding endlessly huge if conditionals?

It's now incredibly hard to track exactly what's going on here, and that is
bug-bait.

>  		init = true;
>  	/*
>  	 * If memory is still not initialized, initialize it now.
> --
> MST
>

Thanks, Lorenzo