Re: Address space operations questions

[Trond Myklebust <trond.myklebust@fys.uio.no>]

to den 31.03.2005 Klokka 13:40 (-0800) skreiv Bryan Henderson:
> >what it
> >*really* means to be called in sync_page() is that you're being told
> >that some process is about to block on that page.  For what reason, you
> >can't know from the call alone.
> 
> Ugh.  IOW it barely means anything.

It reflects the fact that the page lock can be held for a variety of
reasons, some of which require you to kick the filesystem and some which
don't.

I introduced the sync_page() call in 2.4.x partly in order to get rid of
all those pathetic hard-coded calls to "run_task_queue(&tq_disk)" that
used to litter the 2.4.x mm code (and still do in some places). As far
as NFS is concerned, they are a useless distraction since only the block
code uses the tq_disk queue.

The other reason was that the NFS client itself had to defer actually
putting reads on the wire until someone requested the lock: the reason
was that there was no equivalent of the "readpages()" call, so that when
we wanted to coalesce more than 1 page worth of data into a single read
call, we had to exit readpage() without actually starting I/O in the
hope that the readahead code would then schedule a readpage() on a
neighbouring page.

Cheers,
  Trond

-- 
Trond Myklebust <trond.myklebust@fys.uio.no>