linux-kernel - Re: [PATCH v6 1/5] mm/filemap: add mempolicy support to the filemap layer

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Message-ID: <503c4ba7-c667-444a-b396-e85c46469f0a@suse.cz>
Date: Fri, 28 Feb 2025 15:17:11 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Shivank Garg <shivankg@....com>, akpm@...ux-foundation.org,
 willy@...radead.org, pbonzini@...hat.com
Cc: linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 linux-coco@...ts.linux.dev, chao.gao@...el.com, seanjc@...gle.com,
 ackerleytng@...gle.com, david@...hat.com, bharata@....com, nikunj@....com,
 michael.day@....com, Neeraj.Upadhyay@....com, thomas.lendacky@....com,
 michael.roth@....com, tabba@...gle.com
Subject: Re: [PATCH v6 1/5] mm/filemap: add mempolicy support to the filemap
 layer

On 2/26/25 09:25, Shivank Garg wrote:
> From: Shivansh Dhiman <shivansh.dhiman@....com>
> 
> Add NUMA mempolicy support to the filemap allocation path by introducing
> new APIs that take a mempolicy argument:
> - filemap_grab_folio_mpol()
> - filemap_alloc_folio_mpol()
> - __filemap_get_folio_mpol()
> 
> These APIs allow callers to specify a NUMA policy during page cache
> allocations, enabling fine-grained control over memory placement. This is
> particularly needed by KVM when using guest-memfd memory backends, where
> the guest memory needs to be allocated according to the NUMA policy
> specified by VMM.
> 
> The existing non-mempolicy APIs remain unchanged and continue to use the
> default allocation behavior.
> 
> Signed-off-by: Shivansh Dhiman <shivansh.dhiman@....com>
> Signed-off-by: Shivank Garg <shivankg@....com>

<snip>

> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1001,11 +1001,17 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
>  EXPORT_SYMBOL_GPL(filemap_add_folio);
>  
>  #ifdef CONFIG_NUMA
> -struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
> +struct folio *filemap_alloc_folio_mpol_noprof(gfp_t gfp, unsigned int order,
> +		struct mempolicy *mpol)
>  {
>  	int n;
>  	struct folio *folio;
>  
> +	if (mpol)
> +		return folio_alloc_mpol_noprof(gfp, order, mpol,
> +					       NO_INTERLEAVE_INDEX,
> +					       numa_node_id());
> +
>  	if (cpuset_do_page_mem_spread()) {
>  		unsigned int cpuset_mems_cookie;
>  		do {
> @@ -1018,6 +1024,12 @@ struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
>  	}
>  	return folio_alloc_noprof(gfp, order);
>  }
> +EXPORT_SYMBOL(filemap_alloc_folio_mpol_noprof);
> +
> +struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
> +{
> +	return filemap_alloc_folio_mpol_noprof(gfp, order, NULL);
> +}
>  EXPORT_SYMBOL(filemap_alloc_folio_noprof);
>  #endif

Here it seems to me:

- filemap_alloc_folio_noprof() could stay unchanged
- filemap_alloc_folio_mpol_noprof() would
  - call folio_alloc_mpol_noprof() if (mpol)
  - call filemap_alloc_folio_noprof() otherwise

The code would be a bit more clearly structured that way?

> @@ -1881,11 +1893,12 @@ void *filemap_get_entry(struct address_space *mapping, pgoff_t index)
>  }
>  
>  /**
> - * __filemap_get_folio - Find and get a reference to a folio.
> + * __filemap_get_folio_mpol - Find and get a reference to a folio.
>   * @mapping: The address_space to search.
>   * @index: The page index.
>   * @fgp_flags: %FGP flags modify how the folio is returned.
>   * @gfp: Memory allocation flags to use if %FGP_CREAT is specified.
> + * @mpol: The mempolicy to apply when allocating a new folio.
>   *
>   * Looks up the page cache entry at @mapping & @index.
>   *
> @@ -1896,8 +1909,8 @@ void *filemap_get_entry(struct address_space *mapping, pgoff_t index)
>   *
>   * Return: The found folio or an ERR_PTR() otherwise.
>   */
> -struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> -		fgf_t fgp_flags, gfp_t gfp)
> +struct folio *__filemap_get_folio_mpol(struct address_space *mapping, pgoff_t index,
> +		fgf_t fgp_flags, gfp_t gfp, struct mempolicy *mpol)
>  {
>  	struct folio *folio;
>  
> @@ -1967,7 +1980,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  			err = -ENOMEM;
>  			if (order > min_order)
>  				alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
> -			folio = filemap_alloc_folio(alloc_gfp, order);
> +			folio = filemap_alloc_folio_mpol(alloc_gfp, order, mpol);
>  			if (!folio)
>  				continue;
>  
> @@ -2003,6 +2016,13 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  		folio_clear_dropbehind(folio);
>  	return folio;
>  }
> +EXPORT_SYMBOL(__filemap_get_folio_mpol);
> +
> +struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> +		fgf_t fgp_flags, gfp_t gfp)
> +{
> +	return __filemap_get_folio_mpol(mapping, index, fgp_flags, gfp, NULL);
> +}
>  EXPORT_SYMBOL(__filemap_get_folio);
>  
>  static inline struct folio *find_get_entry(struct xa_state *xas, pgoff_t max,