Re: [PATCH 06/12] btrfs: Convert from readpages to readahead

Matthew Wilcox
2020-01-30T08:09:44Z

On Wed, Jan 29, 2020 at 11:46:09AM +1100, Dave Chinner wrote:
> On Fri, Jan 24, 2020 at 05:35:47PM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> >
> > Use the new readahead operation in btrfs
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: linux-btrfs@vger.kernel.org
> > ---
> > fs/btrfs/extent_io.c | 15 ++++-----------
> > fs/btrfs/extent_io.h | 2 +-
> > fs/btrfs/inode.c | 18 +++++++++---------
> > 3 files changed, 14 insertions(+), 21 deletions(-)
> >
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index 2f4802f405a2..b1e2acbec165 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -4283,7 +4283,7 @@ int extent_writepages(struct address_space *mapping,
> > return ret;
> > }
> >
> > -int extent_readpages(struct address_space *mapping, struct list_head *pages,
> > +unsigned extent_readahead(struct address_space *mapping, pgoff_t start,
> > unsigned nr_pages)
> > {
> > struct bio *bio = NULL;
> > @@ -4294,20 +4294,13 @@ int extent_readpages(struct address_space *mapping, struct list_head *pages,
> > int nr = 0;
> > u64 prev_em_start = (u64)-1;
> >
> > - while (!list_empty(pages)) {
> > + while (nr_pages) {
> > u64 contig_end = 0;
> >
> > - for (nr = 0; nr < ARRAY_SIZE(pagepool) && !list_empty(pages);) {
> > - struct page *page = lru_to_page(pages);
> > + for (nr = 0; nr < ARRAY_SIZE(pagepool) && nr_pages--;) {
>
> What is stopping nr_pages from going negative here, and then looping
> forever on the outer nr_pages loop? Perhaps "while(nr_pages > 0) {"
> would be better there?

Ugh, nr_pages is unsigned, so that's no good. Maybe make this a more
conventional loop ...

while (nr_pages) {
u64 contig_end = 0;

for (nr = 0; nr < ARRAY_SIZE(pagepool); nr++) {
struct page *page = readahead_page(mapping, start++);

prefetchw(&page->flags);
pagepool[nr] = page;
contig_end = page_offset(page) + PAGE_SIZE - 1;
if (--nr_pages == 0)
break;
}