Re: [PATCH] remove throttle_vm_writeout()

[Peter Zijlstra]

On Fri, 2007-10-05 at 13:50 -0400, Trond Myklebust wrote:
> On Fri, 2007-10-05 at 12:57 +0200, Peter Zijlstra wrote:
> > In this patch I totally ignored unstable, but I'm not sure that's the
> > proper thing to do, I'd need to figure out what happens to an unstable
> > page when passed into pageout() - or if its passed to pageout at all.
> > 
> > If unstable pages would be passed to pageout(), and it would properly
> > convert them to writeback and clean them, then there is nothing wrong.
> 
> Why would we want to do that? That would be a hell of a lot of work
> (locking pages, setting flags, unlocking pages, ...) for absolutely no
> reason.
> 
> Unstable writes are writes which have been sent to the server, but which
> haven't been written to disk on the server. A single RPC command is then
> sent (COMMIT) which basically tells the server to call fsync(). After
> that is successful, we can free up the pages, but we do that with no
> extra manipulation of the pages themselves: no page locks, just removal
> from the NFS private radix tree, and freeing up of the NFS private
> structures.
> 
> We only need to touch the pages again in the unlikely case that the
> COMMIT fails because the server has rebooted. In this case we have to
> resend the writes, and so the pages are marked as dirty, so we can go
> through the whole writepages() rigmarole again...
> 
> So, no. I don't see sending pages through pageout() as being at all
> helpful.

Well, the thing is, we throttle pageout in throttle_vm_writeout(). As it
stand we can deadlock there because it just waits for the numbers to
drop, and unstable pages don't automagically dissapear. Only
write_inodes() - normally called from balance_dirty_pages() will call
COMMIT.

So my thought was that calling pageout() on an unstable page would do
the COMMIT - we're low on memory, otherwise we would not be paging, so
getting rid of unstable pages seems to make sense to me.



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