Re: [rfc][patch] remove racy sync_page?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:
> On Tue, 30 May 2006, Nick Piggin wrote:
> 
>>But for 2.6.17, how's this?
> 
> 
> It was a great emperor's-clothes-like discovery.  But we've survived
> for so many years without noticing, does it have to be fixed right
> now for 2.6.17?  (I bet I'd be insisting yes if I'd found it.)

It's up to Linus and Andrew I guess. I don't see why not, but I
don't much care one way or the other. But thanks having a quick
look at it, we may want it for the Suse kernel.

> 
> The thing I don't like about your lock_page_nosync (reasonable as
> it is) is that the one case you're using it, set_page_dirty_nolock,
> would be so much happier not to have to lock the page in the first
> place - it's only doing _that_ to stabilize page->mapping, and the
> lock_page forbids it from being called from anywhere that can't
> sleep, which is often just where we want to call it from.  Neil's
> suggestion, using a spin_lock against the mapping changing, would
> help there; but seems like more work than I'd want to get into.

But making PG_lock a spinning lock is completely unrelated to the
bug at hand.

> 
> So, although I think lock_page_nosync fixes the bug (at least in
> that one place we've identified there's likely to be such a bug),
> it seems to be aiming at the wrong target.  I'm pacing and thinking,
> doubt I'll come up with anything better, please don't hold breath.

It is the correct target. I know all about your set_page_dirty_lock
problems, but they aren't what I'm trying to fix.

AFAIKS, you could also make set_page_dirty_lock non sleeping quite
easily by making inode slabs RCU freed.

What places want to use set_page_dirty_lock without sleeping?
The only place in drivers/ apart from sg/st that SetPageDirty are
rd.c and via_dmablit.c, both of which look OK, if a bit crufty.

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