On 2/19/20 1:00 PM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
>
> By reducing nr_to_read, we can eliminate this check from inside the loop.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> mm/readahead.c | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 07cdfbf00f4b..ace611f4bf05 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -166,8 +166,6 @@ void __do_page_cache_readahead(struct address_space *mapping,
> unsigned long lookahead_size)
> {
> struct inode *inode = mapping->host;
> - struct page *page;
> - unsigned long end_index; /* The last page we want to read */
> LIST_HEAD(page_pool);
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> @@ -179,22 +177,27 @@ void __do_page_cache_readahead(struct address_space *mapping,
> ._nr_pages = 0,
> };
> unsigned long i;
> + pgoff_t end_index; /* The last page we want to read */
>
> if (isize == 0)
> return;
>
> - end_index = ((isize - 1) >> PAGE_SHIFT);
> + end_index = (isize - 1) >> PAGE_SHIFT;
> + if (index > end_index)
> + return;
> + if (index + nr_to_read < index)
> + nr_to_read = ULONG_MAX - index + 1;
> + if (index + nr_to_read >= end_index)
> + nr_to_read = end_index - index + 1;
This tiny patch made me pause, because I wasn't sure at first of the exact
intent of the lines above. Once I worked it out, it seemed like it might
be helpful (or overkill??) to add a few hints for the reader, especially since
there are no hints in the function's (minimal) documentation header. What
do you think of this?
/*
* If we can't read *any* pages without going past the inodes's isize
* limit, give up entirely:
*/
if (index > end_index)
return;
/* Cap nr_to_read, in order to avoid overflowing the ULONG type: */
if (index + nr_to_read < index)
nr_to_read = ULONG_MAX - index + 1;
/* Cap nr_to_read, to avoid reading past the inode's isize limit: */
if (index + nr_to_read >= end_index)
nr_to_read = end_index - index + 1;
Either way, it looks corrected written to me, so:
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
thanks,
--
John Hubbard
NVIDIA
>
> /*
> * Preallocate as many pages as we will need.
> */
> for (i = 0; i < nr_to_read; i++) {
> - if (index + i > end_index)
> - break;
> + struct page *page = xa_load(&mapping->i_pages, index + i);
>
> BUG_ON(index + i != rac._index + rac._nr_pages);
>
> - page = xa_load(&mapping->i_pages, index + i);
> if (page && !xa_is_value(page)) {
> /*
> * Page already present? Kick off the current batch of
>
