Re: [patch 5/18] mark swapout pages PageWriteback()

[Pavel Machek]

Hi!

> > > In short the same way MAP_ANON must pageout correctly, also MAP_SHARED
> > > must swapout correctly with very vm intensive conditions.
> 
> I strongly agree with Andrea on that.  Swap fault without page lock
> was a step forward, reinstating it in the light of observed degradation
> in one workload was right decision for that particular tail of release,
> but it's a shame to have stayed that way.
> 
> > That is true, but it is ignoring the fact that there _are_ real technical 
> > differences between swap cache mappings and regular shared mappings.
> > 
> > One major difference is the approach to the last user: a last use of a 
> > shared mapping still needs to write out dirty state, while the last use of 
> > a swap page is better off noticing that it should just optimize away the 
> > write, and we can just turn the page back into a dirty anonymous page.
> 
> Poor example: isn't last use of swap page just like last use of
> shared mapping of an _unlinked_ file?

Eh? Suppose I have two different shared mappings of (unlinked:3). It
may be last use of swap page in first mapping, but what prevents me
from seeking and trying to read what I wrote?
									Pavel

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