Re: [Xen-devel] Re: mapping problems in xenpaging

[Olaf Hering <olaf@xxxxxxxxx>]

On Fri, Sep 30, Adin Scannell wrote:

> >> ÂWhen we analyze and test xenpaging,we found there are some problemsÂ
> >> between
> >> mapping and xenpaging.
> >> Â1) When mapping firstly, then do xenpaging,and the code paths have 
> >> resolved
> >> the problems.It's OK.
> >> Â2) The problems exists if we do address mapping firstly then go to
> >> xenpaging,and our confusions are as followings:
> >> ÂÂ a) If the domU's memory is directly mapped to dom0,such as the hypercall
> >> from pv driver,then it will build a related page-table in dom0,which will 
> >> not
> >> change p2m-type.
> >> ÂÂÂÂ Âand then do the xenpaging to page out the domU's memory pages whose 
> >> gfn
> >> address have been already mapped to dom0;So it will cause some problems 
> >> when
> >> dom0
> >> ÂÂÂÂ Âaccesses these pages.Because these pages are paged-out,and dom0 
> >> cannot
> >> tell the p2mt before access the pages.
> >
> > I'm not entirely sure what you do. xenpaging runs in dom0 and is able to
> > map paged-out pages. It uses that to trigger a page-in, see
> > tools/xenpaging/pagein.c in xen-unstable.hg
> 
> Here's my take...
> 
> Xenpaging doesn't allow selection of pages that have been mapped by
> other domains (as in p2m.c):
> 
>  669 int p2m_mem_paging_nominate(struct domain *d, unsigned long gfn)
> ....
>  693     /* Check page count and type */
>  694     page = mfn_to_page(mfn);
>  695     if ( (page->count_info & (PGC_count_mask | PGC_allocated)) !=
>  696          (1 | PGC_allocated) )
>  697         goto out;
> 
> *However*, I think that the problem Zhen is describing still exists:
> 1) xenpaging nominates a page, it is successful.
> 2) dom0 maps the same page (a process other than xenpaging, which will
> also map it).
> 3) xenpaging evicts the page, successfully.
> 
> I've experienced a few nasty crashes, and I think this could account
> for a couple (but certainly not all)... I think that the solution may
> be to repeat the refcount check in paging_evict, and roll back the
> nomination gracefully if the race is detected. Thoughts?

Are there really code paths that touch a mfn without going through the
p2m functions? If so I will copy the check and update xenpaging.

Olaf

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