Re: [PATCH 36/42] VFS: export drop_pagecache_sb

[Erez Zadok]

In message <200712121638.35167.nickpiggin@xxxxxxxxxxxx>, Nick Piggin writes:
> On Monday 10 December 2007 13:42, Erez Zadok wrote:
> > Needed to maintain cache coherency after branch management.
> >
> 
> Hmm, I'd much prefer to be able to sleep in invalidate_mapping_pages
> before this function gets exported.
> 
> As it is, it can cause massive latencies on preemption and the inode_lock
> so it is pretty much debug-only IMO. I'd rather it didn't escape into the
> wild as is.
> 
> Either that or rework your cache coherency somehow.

Nick, thanks for the advice.

We use a generation number after each successful branch configuration
command, so that ->d_revalidate later on can discover that change, and
rebuild the union of objects.  At ->remount time, I figured it'd be nice to
"encourage" that revalidation to happen sooner, by invalidating as many
upper pages as possible, thus causing ->d_revalidate/->readpage to take
place sooner.  So we used to call drop_pagecache_sb from our remount code:
it was the only caller of drop_pagecache_sb.  It wasn't too much of an
latency issue to call drop_pagecache_sb there: the VFS remount code path is
already pretty slow (dropping temporarily to readonly mode, and dropping
other caches), and remount isn't an operation used often, so a little bit
more latency would probably not have been noticed by users.

Nevertheless, it was not strictly necessary to call drop_pagecache_sb in
unionfs_remount, because the objects in question will have gotten
revalidated sooner or later anyway; the call to drop_pagecache_sb was just
an optimization (one which I wasn't 100% sure about anyway, as per my long
"XXX" comment above that call in unionfs_remount).

So I agree with you: if this symbol can be abused by modules and cause
problems, then exporting it to modules is too risky.  I've reworked my code
to avoid calling drop_pagecache_sb and I'll [sic] drop that patch.

Cheers,
Erez.
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