Subject: Re: [PATCH v10 08/37] mm: add alloc_contig_frozen_pages_user for cache-friendly zeroing

From: Lorenzo Stoakes <[email-masked]>
Date: Mon, 8 Jun 2026 11:29:48 +0100

On Mon, Jun 08, 2026 at 04:35:29AM -0400, Michael S. Tsirkin wrote:
> Add a _user variant of alloc_contig_frozen_pages that accepts a user_addr
> parameter for cache-friendly zeroing of contiguous allocations.
>
> No functional change; all existing callers continue to pass
> USER_ADDR_NONE.

Well, except that you're adding a new function...

>
> Note for reviewers: non-compound contiguous allocations are

Please don't put notes for reviewers in commit messages.

> zeroed via kernel_init_pages, same as before this patch.
> There is no fault address because these allocations are not
> from the page fault path. For compound allocations, user_addr
> reaches post_alloc_hook() which calls folio_zero_user() with
> the dcache flush on cache-aliasing architectures.

Yeah it's exactly this kind of 'just have to know' stuff that makes this
user_addr approach unacceptable.

We mustn't add more cognitive overhead for already confusing code. Now
everybody using these has to figure out what 'user_addr' means and will
inevitably get it wrong.

This whole approach needs to be rethought.

>
> Note about Sashiko (sashiko.dev) false positives: sashiko
> flags two issues here: (1) user_addr silently ignored for
> non-compound allocations, and (2) post_alloc_hook ignores
> user_addr. Both are false positives: (1) non-compound
> contiguous allocations have no fault address to pass, and
> (2) post_alloc_hook does use user_addr when it is not
> USER_ADDR_NONE.

Please don't put AI hallucinations in commit messages.

If you want something to not appear in a commit message, but want reviewers
to see it, put it below the '---' in the patch.

>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>

This patch is unacceptable for the same reasons given for 7/37.

> Assisted-by: Claude:claude-opus-4-6
> ---
>  include/linux/gfp.h |  6 ++++++
>  mm/page_alloc.c     | 42 ++++++++++++++++++++++++++++++++----------
>  2 files changed, 38 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index ee35c5367abc..73109d4e31a4 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -453,6 +453,12 @@ struct page *alloc_contig_frozen_pages_noprof(unsigned long nr_pages,
>  #define alloc_contig_frozen_pages(...) \
>  	alloc_hooks(alloc_contig_frozen_pages_noprof(__VA_ARGS__))
>
> +struct page *alloc_contig_frozen_pages_user_noprof(unsigned long nr_pages,
> +		gfp_t gfp_mask, int nid, nodemask_t *nodemask,
> +		unsigned long user_addr);
> +#define alloc_contig_frozen_pages_user(...) \
> +	alloc_hooks(alloc_contig_frozen_pages_user_noprof(__VA_ARGS__))
> +
>  struct page *alloc_contig_pages_noprof(unsigned long nr_pages, gfp_t gfp_mask,
>  		int nid, nodemask_t *nodemask);
>  #define alloc_contig_pages(...)	\
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 21b52c879751..6d3f284c607d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6975,13 +6975,15 @@ static void __free_contig_frozen_range(unsigned long pfn, unsigned long nr_pages
>  }
>
>  /**
> - * alloc_contig_frozen_range() -- tries to allocate given range of frozen pages
> + * __alloc_contig_frozen_range() -- tries to allocate given range of frozen pages
>   * @start:	start PFN to allocate
>   * @end:	one-past-the-last PFN to allocate
>   * @alloc_flags:	allocation information
>   * @gfp_mask:	GFP mask. Node/zone/placement hints are ignored; only some
>   *		action and reclaim modifiers are supported. Reclaim modifiers
>   *		control allocation behavior during compaction/migration/reclaim.
> + * @user_addr:	user virtual address for cache-friendly zeroing, or
> + *		USER_ADDR_NONE for kernel allocations.

Yeah, I really do not want us passing USER_ADDR_NONE for kernel allocations
thanks. This is hugely confusing already pretty confusing logic.

>   *
>   * The PFN range does not have to be pageblock aligned. The PFN range must
>   * belong to a single zone.
> @@ -6997,8 +6999,9 @@ static void __free_contig_frozen_range(unsigned long pfn, unsigned long nr_pages
>   *
>   * Return: zero on success or negative error code.
>   */
> -int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
> -		acr_flags_t alloc_flags, gfp_t gfp_mask)
> +static int __alloc_contig_frozen_range(unsigned long start, unsigned long end,
> +		acr_flags_t alloc_flags, gfp_t gfp_mask,
> +		unsigned long user_addr)
>  {
>  	const unsigned int order = ilog2(end - start);
>  	unsigned long outer_start, outer_end;
> @@ -7125,7 +7128,7 @@ int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
>  		struct page *head = pfn_to_page(start);
>
>  		check_new_pages(head, order);
> -		prep_new_page(head, order, gfp_mask, 0, USER_ADDR_NONE);
> +		prep_new_page(head, order, gfp_mask, 0, user_addr);
>  	} else {
>  		ret = -EINVAL;
>  		WARN(true, "PFN range: requested [%lu, %lu), allocated [%lu, %lu)\n",
> @@ -7135,6 +7138,13 @@ int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
>  	undo_isolate_page_range(start, end);
>  	return ret;
>  }
> +
> +int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
> +		acr_flags_t alloc_flags, gfp_t gfp_mask)
> +{
> +	return __alloc_contig_frozen_range(start, end, alloc_flags, gfp_mask,
> +					   USER_ADDR_NONE);
> +}
>  EXPORT_SYMBOL(alloc_contig_frozen_range_noprof);
>
>  /**
> @@ -7227,14 +7237,16 @@ static bool zone_spans_last_pfn(const struct zone *zone,
>  	return zone_spans_pfn(zone, last_pfn);
>  }
>
> -/**
> - * alloc_contig_frozen_pages() -- tries to find and allocate contiguous range of frozen pages
> +/*
> + * alloc_contig_frozen_pages_user_noprof() -- allocate contiguous frozen pages with user address
>   * @nr_pages:	Number of contiguous pages to allocate
>   * @gfp_mask:	GFP mask. Node/zone/placement hints limit the search; only some
>   *		action and reclaim modifiers are supported. Reclaim modifiers
>   *		control allocation behavior during compaction/migration/reclaim.
>   * @nid:	Target node
>   * @nodemask:	Mask for other possible nodes
> + * @user_addr:	user virtual address for cache-friendly zeroing, or
> + *		USER_ADDR_NONE for kernel allocations.
>   *
>   * This routine is a wrapper around alloc_contig_frozen_range(). It scans over
>   * zones on an applicable zonelist to find a contiguous pfn range which can then
> @@ -7253,8 +7265,9 @@ static bool zone_spans_last_pfn(const struct zone *zone,
>   *
>   * Return: pointer to contiguous frozen pages on success, or NULL if not successful.
>   */
> -struct page *alloc_contig_frozen_pages_noprof(unsigned long nr_pages,
> -		gfp_t gfp_mask, int nid, nodemask_t *nodemask)
> +struct page *alloc_contig_frozen_pages_user_noprof(unsigned long nr_pages,
> +		gfp_t gfp_mask, int nid, nodemask_t *nodemask,
> +		unsigned long user_addr)
>  {
>  	unsigned long ret, pfn, flags;
>  	struct zonelist *zonelist;
> @@ -7282,10 +7295,11 @@ struct page *alloc_contig_frozen_pages_noprof(unsigned long nr_pages,
>  				 * win the race and cause allocation to fail.
>  				 */
>  				spin_unlock_irqrestore(&zone->lock, flags);
> -				ret = alloc_contig_frozen_range_noprof(pfn,
> +				ret = __alloc_contig_frozen_range(pfn,
>  							pfn + nr_pages,
>  							ACR_FLAGS_NONE,
> -							gfp_mask);
> +							gfp_mask,
> +							user_addr);
>  				if (!ret)
>  					return pfn_to_page(pfn);
>  				spin_lock_irqsave(&zone->lock, flags);
> @@ -7307,6 +7321,14 @@ struct page *alloc_contig_frozen_pages_noprof(unsigned long nr_pages,
>  	}
>  	return NULL;
>  }
> +EXPORT_SYMBOL(alloc_contig_frozen_pages_user_noprof);

Generally we don't add EXPORT_SYMBOL() for new stuff unless
unavoidable. EXPORT_SYMBOL_GPL() is preferred.

> +
> +struct page *alloc_contig_frozen_pages_noprof(unsigned long nr_pages,
> +		gfp_t gfp_mask, int nid, nodemask_t *nodemask)
> +{
> +	return alloc_contig_frozen_pages_user_noprof(nr_pages, gfp_mask, nid,
> +						     nodemask, USER_ADDR_NONE);
> +}
>  EXPORT_SYMBOL(alloc_contig_frozen_pages_noprof);
>
>  /**
> --
> MST
>

Thanks, Lorenzo