[RFC PATCH v4 3/5] mm/readahead: Make space in struct file_ra_state

[Pankaj Raghav (Samsung)]

On Wed, Apr 30, 2025 at 03:59:16PM +0100, Ryan Roberts wrote:
> We need to be able to store the preferred folio order associated with a
> readahead request in the struct file_ra_state so that we can more
> accurately increase the order across subsequent readahead requests. But
> struct file_ra_state is per-struct file, so we don't really want to
> increase it's size.
> 
> mmap_miss is currently 32 bits but it is only counted up to 10 *
> MMAP_LOTSAMISS, which is currently defined as 1000. So 16 bits should be
> plenty. Redefine it to unsigned short, making room for order as unsigned
> short in follow up commit.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>

Looks good.
Reviewed-by: Pankaj Raghav <p.raghav at samsung.com>

> ---
>  include/linux/fs.h |  2 +-
>  mm/filemap.c       | 11 ++++++-----
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 016b0fe1536e..44362bef0010 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1042,7 +1042,7 @@ struct file_ra_state {
>  	unsigned int size;
>  	unsigned int async_size;
>  	unsigned int ra_pages;
> -	unsigned int mmap_miss;
> +	unsigned short mmap_miss;
>  	loff_t prev_pos;
>  };
>  
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 7b90cbeb4a1a..fa129ecfd80f 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3207,7 +3207,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  	DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
>  	struct file *fpin = NULL;
>  	unsigned long vm_flags = vmf->vma->vm_flags;
> -	unsigned int mmap_miss;
> +	unsigned short mmap_miss;
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	/* Use the readahead code, even if readahead is disabled */
> @@ -3275,7 +3275,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>  	struct file_ra_state *ra = &file->f_ra;
>  	DEFINE_READAHEAD(ractl, file, ra, file->f_mapping, vmf->pgoff);
>  	struct file *fpin = NULL;
> -	unsigned int mmap_miss;
> +	unsigned short mmap_miss;
>  
>  	/* If we don't want any read-ahead, don't bother */
>  	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
> @@ -3595,7 +3595,7 @@ static struct folio *next_uptodate_folio(struct xa_state *xas,
>  static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>  			struct folio *folio, unsigned long start,
>  			unsigned long addr, unsigned int nr_pages,
> -			unsigned long *rss, unsigned int *mmap_miss)
> +			unsigned long *rss, unsigned short *mmap_miss)
>  {
>  	vm_fault_t ret = 0;
>  	struct page *page = folio_page(folio, start);
> @@ -3657,7 +3657,7 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>  
>  static vm_fault_t filemap_map_order0_folio(struct vm_fault *vmf,
>  		struct folio *folio, unsigned long addr,
> -		unsigned long *rss, unsigned int *mmap_miss)
> +		unsigned long *rss, unsigned short *mmap_miss)
>  {
>  	vm_fault_t ret = 0;
>  	struct page *page = &folio->page;
> @@ -3699,7 +3699,8 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>  	struct folio *folio;
>  	vm_fault_t ret = 0;
>  	unsigned long rss = 0;
> -	unsigned int nr_pages = 0, mmap_miss = 0, mmap_miss_saved, folio_type;
> +	unsigned int nr_pages = 0, folio_type;
> +	unsigned short mmap_miss = 0, mmap_miss_saved;
>  
>  	rcu_read_lock();
>  	folio = next_uptodate_folio(&xas, mapping, end_pgoff);
> -- 
> 2.43.0
>