Re: [RFC][PATCH v3] readahead: introduce O_RANDOM for POSIX_FADV_RANDOM

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Jan 05, 2010 at 09:46:09AM +0800, Minchan Kim wrote:
> On Mon, Jan 4, 2010 at 9:16 PM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> > Hi Minchan,
> >
> > On Mon, Jan 04, 2010 at 01:20:49PM +0800, Minchan Kim wrote:
> >> > --- linux.orig/mm/readahead.c   2010-01-04 12:39:29.000000000 +0800
> >> > +++ linux/mm/readahead.c        2010-01-04 12:39:30.000000000 +0800
> >> > @@ -501,6 +501,12 @@ void page_cache_sync_readahead(struct ad
> >> >        if (!ra->ra_pages)
> >> >                return;
> >> >
> >> > +       /* be dumb */
> >> > +       if (filp->f_flags & O_RANDOM) {
> >> > +               force_page_cache_readahead(mapping, filp, offset, req_size);
> >> > +               return;
> >> > +       }
> >> > +
> >>
> >> Let me have a dumb question. :)
> >>
> >> How about testing O_RANDOM in front of ra_pages testing?
> >>
> >> My intention is that although we turn off ra, it would be better to read
> >> contiguous block all at once than readpage() callback doing I/O
> >> one page at a time.
> >>
> >> Is it break some semantics or happen some problem in ondemand readahead?
> >
> > Yes it will have some problem with shrink_readahead_size_eio(), which
> > want to disable readahead and use ->readpage() when ra_pages==0.
> >
> > Do you have specific use case in mind? The file systems that set
> > ra_pages=0 seems to don't need readahead, too.
> 
> Never mind. It's just out of curiosity. :)
> 
> I thought although user disable readahead, we could enhance file I/O
> with one readpages not multiple readpage if we know the user want to
> read big contiguous blocks.

Yes, not-break-large-read-into-pages would be good for HD/SSD drives
when readahead is disabled.

Currently, ->ra_pages is somehow overloaded in its ==0 case. As you
said, it's in fact possible to disable readahead while still limiting
read IO size to a non-zero ->ra_pages.

> But I though it break current readahead off semantics. right?

It can be done by applying the ->ra_pages limit to O_RANDOM. This also
makes O_RANDOM safer to use:

@@ -497,6 +497,13 @@ void page_cache_sync_readahead(struct ad
                               struct file_ra_state *ra, struct file *filp,
                               pgoff_t offset, unsigned long req_size)
 {
+       /* be dumb */
+       if (filp->f_flags & O_RANDOM) {
+               req_size = clamp_t(unsigned long, req_size, 1, ra->ra_pages);
+               force_page_cache_readahead(mapping, filp, offset, req_size);
+               return;
+       }
+      
        /* no read-ahead */
        if (!ra->ra_pages)
                return;

To make real change, we need an interface for the user to disable
whole-partition readahead by setting O_RANDOM instead of ra_pages=0.
That would be a hard sell..

> Thanks for reply about my dumb question, Wu. :)

You are welcome :)

Thanks,
Fengguang