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

Milosz Tanski
2015-03-30T23:09:47Z

On Mon, Mar 30, 2015 at 3:36 AM, Christoph Hellwig <hch@infradead.org> 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?
>
>> 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.

I'm okay with an old or nothing flag. Although I think that would much
more useful with RWF_NONWAIT with pwritev, in applications that don't
want to block while logging (but it's okay to drop low level log
messages). That's a whole different use case in my mind.

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Milosz Tanski
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