On Mon, Mar 30, 2015 at 1:19 PM, Jeremy Allison <jra@xxxxxxxxx> wrote: > 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. 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. -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
