Re: [PATCH v9 20/25] ext4: Convert from readpages to readahead

Matthew Wilcox
2020-03-20T17:48:51Z

On Fri, Mar 20, 2020 at 10:37:34AM -0700, Eric Biggers wrote:
> On Fri, Mar 20, 2020 at 07:22:26AM -0700, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> >
> > Use the new readahead operation in ext4
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Reviewed-by: William Kucharski <william.kucharski@oracle.com>
> > ---
> > fs/ext4/ext4.h | 3 +--
> > fs/ext4/inode.c | 21 +++++++++------------
> > fs/ext4/readpage.c | 22 ++++++++--------------
> > 3 files changed, 18 insertions(+), 28 deletions(-)
> >
>
> Reviewed-by: Eric Biggers <ebiggers@google.com>
>
> > + if (rac) {
> > + page = readahead_page(rac);
> > prefetchw(&page->flags);
> > - list_del(&page->lru);
> > - if (add_to_page_cache_lru(page, mapping, page->index,
> > - readahead_gfp_mask(mapping)))
> > - goto next_page;
> > }
>
> Maybe the prefetchw(&page->flags) should be included in readahead_page()?
> Most of the callers do it.

I did notice that a lot of callers do that. I wonder whether it (still)
helps or whether it's just cargo-cult programming. It can't possibly
have helped before because we did list_del(&page->lru) as the very next
instruction after prefetchw(), and they're in the same cacheline. It'd
be interesting to take it out and see what happens to performance.