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

On Mon, Mar 30, 2015 at 1:19 PM, Jeremy Allison [off-list ref] wrote:

On Mon, Mar 30, 2015 at 12:36:04AM -0700, Christoph Hellwig wrote:

quoted

On Fri, Mar 27, 2015 at 08:58:54AM -0700, Jeremy Allison wrote:

quoted

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.

quoted

quoted

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.

The partial behavior is very useful for protocols like HTTP1, since
the client can start processing the response if we send it down the
wire while we process other connections. It becomes even more useful
over HTTP2 which provides it's own framing where we can send a partial
response frame and move onto other requests in this connection or
other connections.

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