Re: [PATCH v2 02/18] mm/filemap: Remove dynamically allocated array from filemap_read

[Kent Overstreet <kent.overstreet@gmail.com>]

On Wed, Nov 04, 2020 at 08:42:03PM +0000, Matthew Wilcox (Oracle) wrote:
> Increasing the batch size runs into diminishing returns.  It's probably
> better to make, eg, three calls to filemap_get_pages() than it is to
> call into kmalloc().

I have to disagree. Working with PAGEVEC_SIZE pages is eventually going to be
like working with 4k pages today, and have you actually read the slub code for
the kmalloc fast path? It's _really_ fast, there's no atomic operations and it
doesn't even have to disable preemption - which is why you never see it showing
up in profiles ever since we switched to slub.

It would however be better to have a standard abstraction for this rather than
open coding it - perhaps adding it to the pagevec code. Please don't just drop
it, though.


> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/filemap.c | 15 ++-------------
>  1 file changed, 2 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 23e3781b3aef..bb1c42d0223c 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2429,8 +2429,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  	struct file_ra_state *ra = &filp->f_ra;
>  	struct address_space *mapping = filp->f_mapping;
>  	struct inode *inode = mapping->host;
> -	struct page *pages_onstack[PAGEVEC_SIZE], **pages = NULL;
> -	unsigned int nr_pages = min_t(unsigned int, 512,
> +	struct page *pages[PAGEVEC_SIZE];
> +	unsigned int nr_pages = min_t(unsigned int, PAGEVEC_SIZE,
>  			((iocb->ki_pos + iter->count + PAGE_SIZE - 1) >> PAGE_SHIFT) -
>  			(iocb->ki_pos >> PAGE_SHIFT));
>  	int i, pg_nr, error = 0;
> @@ -2441,14 +2441,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  		return 0;
>  	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
>  
> -	if (nr_pages > ARRAY_SIZE(pages_onstack))
> -		pages = kmalloc_array(nr_pages, sizeof(void *), GFP_KERNEL);
> -
> -	if (!pages) {
> -		pages = pages_onstack;
> -		nr_pages = min_t(unsigned int, nr_pages, ARRAY_SIZE(pages_onstack));
> -	}
> -
>  	do {
>  		cond_resched();
>  
> @@ -2533,9 +2525,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  
>  	file_accessed(filp);
>  
> -	if (pages != pages_onstack)
> -		kfree(pages);
> -
>  	return written ? written : error;
>  }
>  EXPORT_SYMBOL_GPL(generic_file_buffered_read);
> -- 
> 2.28.0
>