linux-kernel - Re: [PATCH] kvm/arm: return 0 when the number of objects is not lessthan min

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Message-ID: <20181206142932.kmt3srhncfpyznu5@kamzik.brq.redhat.com>
Date:   Thu, 6 Dec 2018 15:29:32 +0100
From:   Andrew Jones <drjones@...hat.com>
To:     peng.hao2@....com.cn
Cc:     christoffer.dall@....com, marc.zyngier@....com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/arm: return 0 when the number of objects is not
 lessthan min

On Thu, Dec 06, 2018 at 09:56:30AM +0800, peng.hao2@....com.cn wrote:
> >On Wed, Dec 05, 2018 at 09:15:51AM +0800, Peng Hao wrote:
> >> Return 0 when there is enough kvm_mmu_memory_cache object.
> >>
> >> Signed-off-by: Peng Hao <peng.hao2@....com.cn>
> >> ---
> >>  virt/kvm/arm/mmu.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> >> index ed162a6..fcda0ce 100644
> >> --- a/virt/kvm/arm/mmu.c
> >> +++ b/virt/kvm/arm/mmu.c
> >> @@ -127,7 +127,7 @@ static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,
> >>      while (cache->nobjs < max) {
> >>          page = (void *)__get_free_page(PGALLOC_GFP);
> >>          if (!page)
> >> -            return -ENOMEM;
> >> +            return cache->nobjs >= min ? 0 : -ENOMEM;
> >
> >This condition will never be true here, as the exact same condition is
> >already checked above, and if it had been true, then we wouldn't be here.
> >
> >What problem are you attempting to solve?
> >
> if (cache->nobjs >= min)                      ------here cache->nobjs<min,it can continue downward 
>      return 0;
> while (cache->nobjs < max) {
>     page = (void *)__get_free_page(PGALLOC_GFP);
>     if (!page)
>        return -ENOMEM;                         -----here it is possible that  (cache->nobjs >= min) and (cache->nobjs<max)
>     cache->objects[cache->nobjs++] = page; ---here cache->nobjs increasing
>   }
> 
> I just think the logic of this function is to return 0 as long as (cache->nobjs >= min).
> thanks.

Oh, I see now. This is the case where we can do enough allocating to over
the min line, but fail before we get to the max.

Thanks,
drew