Re: [Xen-devel] [PATCHv2 2/3] mm: don't free pages until mm locks are released

[Tim Deegan <tim@xxxxxxx>]

At 07:25 +0000 on 02 Dec (1449041100), Tian, Kevin wrote:
> > From: David Vrabel [mailto:david.vrabel@xxxxxxxxxx]
> > Sent: Saturday, November 14, 2015 2:50 AM
> > 
> > If a page is freed without translations being invalidated, and the page is
> > subsequently allocated to another domain, a guest with a cached
> > translation will still be able to access the page.
> > 
> > Currently translations are invalidated before releasing the page ref, but
> > while still holding the mm locks.  To allow translations to be invalidated
> > without holding the mm locks, we need to keep a reference to the page
> > for a bit longer in some cases.
> > 
> > [ This seems difficult to a) verify as correct; and b) difficult to get
> > correct in the future.  A better suggestion would be useful.  Perhaps
> > using something like pg->tlbflush_needed mechanism that already exists
> > for pages from PV guests? ]
> 
> Per-page flag looks clean in general, but not an expert here. Tim might
> have a better idea.

I think you can probably use the tlbflush_timestamp stuff as-is for
EPT flushes -- the existing TLB shootdowns already drop all EPT
translations.

Holding the ref until the flush finishes is a common enough idiom but
I can see that having to track lists of them does make it a little clunky. 

Cheers,

Tim.

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