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Date: Sun, 11 Aug 2002 20:28:12 -0700
From: Andrew Morton <akpm@zip.com.au>
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To: Linus Torvalds <torvalds@transmeta.com>
CC: Simon Kirby <sim@netnation.com>, linux-kernel@vger.kernel.org,
       Jens Axboe <axboe@suse.de>,
       Trond Myklebust <trond.myklebust@fys.uio.no>
Subject: Re: [patch 6/12] hold atomic kmaps across generic_file_read
References: <20020811031705.GA13878@netnation.com> <Pine.LNX.4.44.0208111121510.9930-100000@home.transmeta.com>
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Linus Torvalds wrote:
> 
> ...
> Basically, it _used_ to be that each page got woken up one at a time as
> they became ready after IO. With the new scheme, they all get woken up
> together in "mpage_end_io_read()" (or write, but since people usually
> don't wait for writes..).
> 
> At least that is how I read the code. Andrew?

Yes.  The basic unit of IO in there is a 64k BIO.  So once readahead
is cruising, pages come unlocked in 16-page batches.  In 2.4 they'll
come unlocked one at a time against a device such as a floppy drive.

But with default settings the readahead code lays one to two of these
BIOs out ahead of the read point, so the application never stumbles across
a locked page unless it's outpacing the device.

At least that's the theory, and the testing I did yesterday
was succesful.

So I'd appreciate it if Simon could invetigate a little further
with the test app I posted.  Something is up, and it may not
be just an NFS thing.  But note that nfs_readpage will go
synchronous if rsize is less than PAGE_CACHE_SIZE, so it has
to be set up right.
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