linux-kernel - Re: [PATCH net-next v9 10/13] mm: page_frag: introduce prepare/probe/commit API

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Date: Fri, 28 Jun 2024 15:35:04 -0700
From: Alexander H Duyck <alexander.duyck@...il.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
 kuba@...nel.org,  pabeni@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Andrew Morton
	 <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH net-next v9 10/13] mm: page_frag: introduce
 prepare/probe/commit API

On Tue, 2024-06-25 at 21:52 +0800, Yunsheng Lin wrote:
> There are many use cases that need minimum memory in order
> for forward progress, but more performant if more memory is
> available or need to probe the cache info to use any memory
> available for frag caoleasing reason.
> 
> Currently skb_page_frag_refill() API is used to solve the
> above use cases, but caller needs to know about the internal
> detail and access the data field of 'struct page_frag' to
> meet the requirement of the above use cases and its
> implementation is similar to the one in mm subsystem.
> 
> To unify those two page_frag implementations, introduce a
> prepare API to ensure minimum memory is satisfied and return
> how much the actual memory is available to the caller and a
> probe API to report the current available memory to caller
> without doing cache refilling. The caller needs to either call
> the commit API to report how much memory it actually uses, or
> not do so if deciding to not use any memory.
> 
> As next patch is about to replace 'struct page_frag' with
> 'struct page_frag_cache' in linux/sched.h, which is included
> by the asm-offsets.s, using the virt_to_page() in the inline
> helper of page_frag_cache.h cause a "'vmemmap' undeclared"
> compiling error for asm-offsets.s, use a macro for probe API
> to avoid that compiling error.
> 
> CC: Alexander Duyck <alexander.duyck@...il.com>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
>  include/linux/page_frag_cache.h |  82 +++++++++++++++++++++++
>  mm/page_frag_cache.c            | 114 ++++++++++++++++++++++++++++++++
>  2 files changed, 196 insertions(+)
> 
> diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
> index b33904d4494f..e95d44a36ec9 100644
> --- a/include/linux/page_frag_cache.h
> +++ b/include/linux/page_frag_cache.h
> @@ -4,6 +4,7 @@
>  #define _LINUX_PAGE_FRAG_CACHE_H
>  
>  #include <linux/gfp_types.h>
> +#include <linux/mmdebug.h>
>  
>  #define PAGE_FRAG_CACHE_MAX_SIZE	__ALIGN_MASK(32768, ~PAGE_MASK)
>  #define PAGE_FRAG_CACHE_MAX_ORDER	get_order(PAGE_FRAG_CACHE_MAX_SIZE)
> @@ -87,6 +88,9 @@ static inline unsigned int page_frag_cache_page_size(struct encoded_va *encoded_
>  
>  void page_frag_cache_drain(struct page_frag_cache *nc);
>  void __page_frag_cache_drain(struct page *page, unsigned int count);
> +struct page *page_frag_alloc_pg(struct page_frag_cache *nc,
> +				unsigned int *offset, unsigned int fragsz,
> +				gfp_t gfp);
>  void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
>  				 unsigned int fragsz, gfp_t gfp_mask,
>  				 unsigned int align_mask);
> @@ -99,12 +103,90 @@ static inline void *page_frag_alloc_va_align(struct page_frag_cache *nc,
>  	return __page_frag_alloc_va_align(nc, fragsz, gfp_mask, -align);
>  }
>  
> +static inline unsigned int page_frag_cache_page_offset(const struct page_frag_cache *nc)
> +{
> +	return page_frag_cache_page_size(nc->encoded_va) - nc->remaining;
> +}
> +
>  static inline void *page_frag_alloc_va(struct page_frag_cache *nc,
>  				       unsigned int fragsz, gfp_t gfp_mask)
>  {
>  	return __page_frag_alloc_va_align(nc, fragsz, gfp_mask, ~0u);
>  }
>  
> +void *page_frag_alloc_va_prepare(struct page_frag_cache *nc, unsigned int *fragsz,
> +				 gfp_t gfp);
> +
> +static inline void *page_frag_alloc_va_prepare_align(struct page_frag_cache *nc,
> +						     unsigned int *fragsz,
> +						     gfp_t gfp,
> +						     unsigned int align)
> +{
> +	WARN_ON_ONCE(!is_power_of_2(align) || align > PAGE_SIZE);
> +	nc->remaining = nc->remaining & -align;
> +	return page_frag_alloc_va_prepare(nc, fragsz, gfp);
> +}
> +
> +struct page *page_frag_alloc_pg_prepare(struct page_frag_cache *nc,
> +					unsigned int *offset,
> +					unsigned int *fragsz, gfp_t gfp);
> +
> +struct page *page_frag_alloc_prepare(struct page_frag_cache *nc,
> +				     unsigned int *offset,
> +				     unsigned int *fragsz,
> +				     void **va, gfp_t gfp);
> +
> +static inline struct encoded_va *__page_frag_alloc_probe(struct page_frag_cache *nc,
> +							 unsigned int *offset,
> +							 unsigned int *fragsz,
> +							 void **va)
> +{
> +	struct encoded_va *encoded_va;
> +
> +	*fragsz = nc->remaining;
> +	encoded_va = nc->encoded_va;
> +	*offset = page_frag_cache_page_size(encoded_va) - *fragsz;
> +	*va = encoded_page_address(encoded_va) + *offset;
> +
> +	return encoded_va;
> +}
> +
> +#define page_frag_alloc_probe(nc, offset, fragsz, va)			\
> +({									\
> +	struct page *__page = NULL;					\
> +									\
> +	VM_BUG_ON(!*(fragsz));						\
> +	if (likely((nc)->remaining >= *(fragsz)))			\
> +		__page = virt_to_page(__page_frag_alloc_probe(nc,	\
> +							      offset,	\
> +							      fragsz,	\
> +							      va));	\
> +									\
> +	__page;								\
> +})
> +

Why is this a macro instead of just being an inline? Are you trying to
avoid having to include a header due to the virt_to_page?