linux-kernel - Re: [PATCH 2/7] mm: Rewrite shmem_seek_hole

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Date:   Thu, 20 Aug 2020 19:45:46 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] mm: Rewrite shmem_seek_hole_data

On Wed, Aug 19, 2020 at 04:05:50PM +0100, Matthew Wilcox (Oracle) wrote:
> use the XArray directly instead of using the pagevec abstraction.
> The code is simpler and more efficient.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/shmem.c | 61 +++++++++++++++++++++---------------------------------
>  1 file changed, 24 insertions(+), 37 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index a7bbc4ed9677..0f9f149f4b5e 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2659,53 +2659,40 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  }
>  
>  /*
> - * llseek SEEK_DATA or SEEK_HOLE through the page cache.
> + * llseek SEEK_DATA or SEEK_HOLE through the page cache.  We don't need
> + * to get a reference on the page because this interface is racy anyway.
> + * The page we find will have had the state at some point.

For my non-native ear "will have had" is too complex ;-)

>   */
>  static pgoff_t shmem_seek_hole_data(struct address_space *mapping,
>  				    pgoff_t index, pgoff_t end, int whence)
>  {
> +	XA_STATE(xas, &mapping->i_pages, index);
>  	struct page *page;
> -	struct pagevec pvec;
> -	pgoff_t indices[PAGEVEC_SIZE];
> -	bool done = false;
> -	int i;
>  
> -	pagevec_init(&pvec);
> -	pvec.nr = 1;		/* start small: we may be there already */
> -	while (!done) {
> -		pvec.nr = find_get_entries(mapping, index,
> -					pvec.nr, pvec.pages, indices);
> -		if (!pvec.nr) {
> -			if (whence == SEEK_DATA)
> -				index = end;
> -			break;
> +	rcu_read_lock();
> +	if (whence == SEEK_DATA) {
> +		for (;;) {
> +			page = xas_find(&xas, end);
> +			if (xas_retry(&xas, page))
> +				continue;
> +			if (!page || xa_is_value(page) || PageUptodate(page))
> +				break;
>  		}
> -		for (i = 0; i < pvec.nr; i++, index++) {
> -			if (index < indices[i]) {
> -				if (whence == SEEK_HOLE) {
> -					done = true;
> -					break;
> -				}
> -				index = indices[i];
> -			}
> -			page = pvec.pages[i];
> -			if (page && !xa_is_value(page)) {
> -				if (!PageUptodate(page))
> -					page = NULL;
> -			}
> -			if (index >= end ||
> -			    (page && whence == SEEK_DATA) ||
> -			    (!page && whence == SEEK_HOLE)) {
> -				done = true;
> +	} else /* SEEK_HOLE */ {
> +		for (;;) {
> +			page = xas_next(&xas);
> +			if (xas_retry(&xas, page))
> +				continue;
> +			if (!xa_is_value(page) &&
> +					(!page || !PageUptodate(page)))
> +				break;
> +			if (xas.xa_index >= end)
>  				break;
> -			}
>  		}
> -		pagevec_remove_exceptionals(&pvec);
> -		pagevec_release(&pvec);
> -		pvec.nr = PAGEVEC_SIZE;
> -		cond_resched();
>  	}
> -	return index;
> +	rcu_read_unlock();
> +
> +	return xas.xa_index;
>  }
>  
>  static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
> -- 
> 2.28.0
> 
> 

-- 
Sincerely yours,
Mike.