Re: [Xen-devel] [PATCH] x86/shadow: don't enable shadow mode with too small a shadow allocation (part 2)

[Jan Beulich <jbeulich@suse.com>]

On 04.09.2019 13:28, Andrew Cooper wrote:
> On 04/09/2019 08:55, Jan Beulich wrote:
>> Commit 2634b997af ("x86/shadow: don't enable shadow mode with too small
>> a shadow allocation") was incomplete: The adjustment done there to
>> shadow_enable() is also needed in shadow_one_bit_enable(). The (new)
>> problem report was (apparently) a failed PV guest migration followed by
>> another migration attempt for that same guest. Disabling log-dirty mode
>> after the first one had left a couple of shadow pages allocated (perhaps
>> something that also wants fixing), and hence the second enabling of
>> log-dirty mode wouldn't have allocated anything further.
>>
>> Reported-by: James Wang <jnwang@suse.com>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>
>> --- a/xen/arch/x86/mm/shadow/common.c
>> +++ b/xen/arch/x86/mm/shadow/common.c
>> @@ -2864,7 +2864,8 @@ static int shadow_one_bit_enable(struct
>>  
>>      mode |= PG_SH_enable;
>>  
>> -    if ( d->arch.paging.shadow.total_pages == 0 )
>> +    if ( d->arch.paging.shadow.total_pages <
>> +         sh_min_allocation(d) + d->arch.paging.shadow.p2m_pages )
> 
> This logic looks suspect.  The change from == 0 to < min looks fine, and
> feels like the right thing to do.
> 
> However,  sh_min_allocation() should by definition be the minimum
> quantity of pages necessary for things to function, which makes the + on
> the end look wrong.

Well, the change here brings shadow_one_bit_enable() in line with
shadow_enable(). What you suggest is a 2nd change, also requiring
an adjustment to shadow_set_allocation(). Back when putting
together 2634b997af for some reason I thought it would be
correct to move the p2m_pages addition into sh_min_allocation(),
but looking again now I think this ought to be quite fine.

There's a possible argument against doing this (or against it
being correct), though: When p2m_pages is already non-zero, why
would it be correct for sh_min_allocation() to add in that value
_and_ also account for to-be-allocated P2M pages itself?
Shouldn't it then rather be the maximum of the current and
prospected values? (To me this is a clear argument for not
folding in such an adjustment into the patch here.)

Jan

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