Re: [patch][rfc] mm: hold page lock over page_mkwrite

[Nick Piggin <npiggin@suse.de>]

On Wed, Feb 25, 2009 at 08:42:50AM -0800, Zach Brown wrote:
> Nick Piggin wrote:
> > I want to have the page be protected by page lock between page_mkwrite
> > notification to the filesystem, and the actual setting of the page
> > dirty. Do this by holding the page lock over page_mkwrite, and keep it
> > held until after set_page_dirty.
> 
> I fear that this will end up creating lock inversions with file systems
> who grab cross-node locks, which are ordered outside of the page lock,
> inside their ->page_mkwrite().  See ocfs2's call of ocfs2_inode_lock()
> from ocfs2_page_mkwrite().

Is ocfs2 immune to the races that get covered by this patch?

 
> In a sense, it's prepare_write() all over again.  Please don't call into
> file systems after having acquired the page lock.

It is very much an opt-out thing with page_mkwrite. The filesystem
if it thinks it is really smart can unlock the page if it likes. I'm
sure it will probably introduce some obscure race or another, but hey ;)


> Instead, can we get a helper that the file system would call after
> having acquired its locks and the page lock?  Something like that.

Well, the critical part is holding the same page lock over the
"important" part of the page_mkwrite operation and setting the
pte dirty and marking the page dirty. So there isn't really a
helper it can call.

Hmm, actually possibly we can enter page_mkwrite with the page unlocked,
but exit with the page locked? Slightly more complex, but should save
complexity elsewhere. Yes I think this might be the best way to go.

Thanks,
Nick

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