On Wed, 11 Oct 2006, Nick Piggin wrote:
> >
> > The original IO could have been started by a person who didn't have
> > permissions to actually carry it out successfully, so if you enter with
> > the page locked (because somebody else started the IO), and you wait for
> > the page and it's not up-to-date afterwards, you absolutely _have_ to try
> > the IO, and can only return a real IO error after your _own_ IO has
> > failed.
>
> Sure, but we currently try to read _twice_, don't we?
Well, we have the read-ahead, and then the real read. By the time we do
the real read, we have forgotten about the read-ahead details, so..
We also end up often having a _third_ one, simply because the _user_ tries
it twice: it gets a partial IO read first, and then tries to continue and
won't give up until it gets a real error.
So yes, we can end up reading it even more than twice, if only due to
standard UNIX interfaces: you always have to have one extra "read()"
system call in order to get the final error (or - much more commonly -
EOF, of course).
If we tracked the read-aheads that _we_ started, we could probably get rid
of one of them.
Linus
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