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

[Volker Lendecke]

On Fri, Mar 27, 2015 at 02:01:59AM -0700, Andrew Morton wrote:
> On Fri, 27 Mar 2015 01:48:33 -0700 Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.

Sorry, but I don't have a good picture how we are supposed
to use that. I'm fine with two syscalls, but I need a way to
tell the kernel to either block or not. Or do you want Samba
to do repeated pread calls for ever shorter blocks? Right
now I don't see a way to tell pread to either give me a
short result or really block. To me that's the core of
preadv2. I'm perfectly find for a syscall to give me a short
read instead of a global EWOULDBLOCK. I need a way to tell
the kernel which behaviour I want.

Volker

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