Re: [Xen-devel] [PATCH v8 3/4] mm: make pages allocated with MEMF_no_refcount safe to assign

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.01.2020 15:57, Paul Durrant wrote:
> Currently it is unsafe to assign a domheap page allocated with
> MEMF_no_refcount to a domain because the domain't 'tot_pages' will not
> be incremented, but will be decrement when the page is freed (since
> free_domheap_pages() has no way of telling that the increment was skipped).
> 
> This patch allocates a new 'count_info' bit for a PGC_extra flag
> which is then used to mark pages when alloc_domheap_pages() is called
> with MEMF_no_refcount. assign_pages() because it still needs to call
> domain_adjust_tot_pages() to make sure the domain is appropriately
> referenced. Hence it is modified to do that for PGC_extra pages even if it
> is passed MEMF_no_refount.
> 
> The number of PGC_extra pages assigned to a domain is tracked in a new
> 'extra_pages' counter, which is then subtracted from 'total_pages' in
> the domain_tot_pages() helper. Thus 'normal' page assignments will still
> be appropriately checked against 'max_pages'.
> 
> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

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