Re: Setting of the PageReadahed bit

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

On Sat, Jun 04, 2011 at 11:15:38AM +0800, Hugh Dickins wrote:
> On Fri, Jun 3, 2011 at 4:55 AM, Matthew Wilcox <willy@linux.intel.com> wrote:
> > The exact definition of PageReadahead doesn't seem to be documented
> > anywhere. A I'm assuming it means "This page was not directly requested;
> > it is being read for prefetching purposes", exactly like the READA
> > semantics.
> >
> > If my interpretation is correct, then the implementation in
> > __do_page_cache_readahead is wrong:
> >
> > A  A  A  A  A  A  A  A if (page_idx == nr_to_read - lookahead_size)
> > A  A  A  A  A  A  A  A  A  A  A  A SetPageReadahead(page);
> >
> > It'll only set the PageReadahead bit on one page. A The patch below fixes
> > this ... if my understanding is correct.
> 
> Incorrect I believe: it's a trigger to say, when you get this far,
> it's time to think about kicking off the next read.

That's right. PG_readahead is set to trigger the _next_ ASYNC readahead.

> >
> > If my understanding is wrong, then how are readpage/readpages
> > implementations supposed to know that the VM is only prefetching these
> > pages, and they're not as important as metadata (dependent) reads?
> 
> I don't think they do know at present; but I can well imagine there
> may be advantage in them knowing.

__do_page_cache_readahead() don't know whether the _current_ readahead
IO is an ASYNC one.

page_cache_async_readahead() calls ondemand_readahead() with
hit_readahead_marker=true. It's possible to further pass this
information into __do_page_cache_readahead() and ->readpage/readpages.

Thanks,
Fengguang

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