Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only)

Andrew Morton
2015-03-27T09:00:54Z

On Fri, 27 Mar 2015 01:48:33 -0700 Christoph Hellwig <hch@infradead.org> wrote:

> On Fri, Mar 27, 2015 at 01:35:16AM -0700, Andrew Morton wrote:
> > fincore() doesn't have to be ugly. Please address the design issues I
> > raised. How is pread2() useful to the class of applications which
> > cannot proceed until all data is available?
>
> It actually makes them work correctly? preadv2( ..., DONTWAIT) will
> return -EGAIN, which causes them to bounce to the threadpool where
> they call preadv(...).

(I assume you mean RWF_NONBLOCK)

That isn't how pread2() works. If the leading one or more pages are
uptodate, pread2() will return a partial read. Now what? Either the
application reads the same data a second time via the worker thread
(dumb, but it will usually be a rare case) or it reads the remainder of
the data in the worker thread and splices the data back together.
Which, as I said, will often result in a second load of the initial
read result into CPU cache.