Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

[Ni zhan Chen]

On 10/26/2012 09:27 AM, Fengguang Wu wrote:
> On Fri, Oct 26, 2012 at 11:25:44AM +1100, Dave Chinner wrote:
> > On Thu, Oct 25, 2012 at 10:58:26AM +0800, Fengguang Wu wrote:
> > > Hi Chen,
> > >
> > > > But how can bdi related ra_pages reflect different files' readahead
> > > > window? Maybe these different files are sequential read, random read
> > > > and so on.
> > > It's simple: sequential reads will get ra_pages readahead size while
> > > random reads will not get readahead at all.
> > >
> > > Talking about the below chunk, it might hurt someone that explicitly
> > > takes advantage of the behavior, however the ra_pages*2 seems more
> > > like a hack than general solution to me: if the user will need
> > > POSIX_FADV_SEQUENTIAL to double the max readahead window size for
> > > improving IO performance, then why not just increase bdi->ra_pages and
> > > benefit all reads? One may argue that it offers some differential
> > > behavior to specific applications, however it may also present as a
> > > counter-optimization: if the root already tuned bdi->ra_pages to the
> > > optimal size, the doubled readahead size will only cost more memory
> > > and perhaps IO latency.
> > >
> > > --- a/mm/fadvise.c
> > > +++ b/mm/fadvise.c
> > > @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
> > >                  spin_unlock(&file->f_lock);
> > >                  break;
> > >          case POSIX_FADV_SEQUENTIAL:
> > > -               file->f_ra.ra_pages = bdi->ra_pages * 2;
> > I think we really have to reset file->f_ra.ra_pages here as it is
> > not a set-and-forget value. e.g.  shrink_readahead_size_eio() can
> > reduce ra_pages as a result of IO errors. Hence if you have had io
> > errors, telling the kernel that you are now going to do  sequential
> > IO should reset the readahead to the maximum ra_pages value
> > supported....
> Good point!
>
> .... but wait .... this patch removes file->f_ra.ra_pages in all other
> places too, so there will be no file->f_ra.ra_pages to be reset here...

In his patch,

 static void shrink_readahead_size_eio(struct file *filp,
                                        struct file_ra_state *ra)
 {
-       ra->ra_pages /= 4;
+       spin_lock(&filp->f_lock);
+       filp->f_mode |= FMODE_RANDOM;
+       spin_unlock(&filp->f_lock);

As the example in comment above this function, the read maybe still 
sequential, and it will waste IO bandwith if modify to FMODE_RANDOM 
directly.

> Thanks,
> Fengguang

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