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

Jeremy Allison
2015-03-30T17:19:19Z

On Mon, Mar 30, 2015 at 12:36:04AM -0700, Christoph Hellwig wrote:
> On Fri, Mar 27, 2015 at 08:58:54AM -0700, Jeremy Allison wrote:
> > The problem with the above is that we can't tell the difference
> > between pread2() returning a short read because the pages are not
> > in cache, or because someone truncated the file. So we need some
> > way to differentiate this.
>
> Is a race vs truncate really that time critical that you can't
> wait for the thread pool to do the second read to notice it?

Probably not, as this is the fallback path anyway.

> > My preference from userspace would be for pread2() to return
> > EAGAIN if *all* the data requested is not available (where
> > 'all' can be less than the size requested if the file has
> > been truncated in the meantime).
>
> That is easily implementable, but I can see that for example web apps
> would be happy to get as much as possible. So if Samba can be ok
> with short reads and only detecting the truncated case in the slow
> path that would make life simpler. Otherwise we might indeed need two
> flags.

Simpler is better. I can live with the partial read+fallback.

Jeremy.