Re: [PATCH 1/3] mpage: mpage_readpages() should submit IO as read-ahead

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 29 May 2018 08:36:41 -0600 Jens Axboe <axboe@kernel.dk> wrote:

> On 5/24/18 1:43 PM, Andrew Morton wrote:
> > On Thu, 24 May 2018 10:02:52 -0600 Jens Axboe <axboe@kernel.dk> wrote:
> > 
> >> a_ops->readpages() is only ever used for read-ahead, yet we don't
> >> flag the IO being submitted as such. Fix that up. Any file system
> >> that uses mpage_readpages() as it's ->readpages() implementation
> >> will now get this right.
> >>
> >> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >> ---
> >>  fs/mpage.c | 17 +++++++++--------
> >>  1 file changed, 9 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/fs/mpage.c b/fs/mpage.c
> >> index b7e7f570733a..0a5474237f5e 100644
> >> --- a/fs/mpage.c
> >> +++ b/fs/mpage.c
> >> @@ -146,7 +146,7 @@ static struct bio *
> >>  do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
> >>  		sector_t *last_block_in_bio, struct buffer_head *map_bh,
> >>  		unsigned long *first_logical_block, get_block_t get_block,
> >> -		gfp_t gfp)
> >> +		gfp_t gfp, bool is_readahead)
> > 
> > That's a lot of arguments.
> > 
> > I suspect we'll have a faster kernel if we mark this __always_inline. 
> > I think my ancient "This isn't called much at all" over
> > mpage_readpage() remains true.  Almost all callers come in via
> > mpage_readpages(), which would benefit from the inlining.  But mpage.o
> > gets 1.5k fatter.  hm.
> 
> Was going to send out a v2, but would be nice to get some consensus on
> what you prefer here. I can either do the struct version, or I can
> keep it as-is (going from 8 to 9 arguments). For the struct version,
> I'd prefer to do that as a prep patch, so the functional change is
> clear.

The struct thing makes the code smaller, and presumably faster, doesn't
it?  I suppose it saves a bit of stack as well, by letting the callee
access the caller's locals rather than a copy of them.  All sounds good
to me?