Re: [PATCH 01/10] net: Use cached copy of pfmemalloc to avoid accessing page

Alexander Duyck
2015-05-11T00:01:15Z

On 05/10/2015 04:18 PM, David Miller wrote:
> From: Alexander Duyck <alexander.h.duyck@redhat.com>
> Date: Wed, 06 May 2015 21:11:40 -0700
>
>> + /* use OR instead of assignment to avoid clearing of bits in mask */
>> + if (pfmemalloc)
>> + skb->pfmemalloc = 1;
>> + skb->head_frag = 1;
> ...
>> + /* use OR instead of assignment to avoid clearing of bits in mask */
>> + if (nc->pfmemalloc)
>> + skb->pfmemalloc = 1;
>> + skb->head_frag = 1;
> Maybe make these two cases more consistent by either accessing
> nc->pfmemalloc or using a local variable in both cases.

The only option would be to use a local variable in both cases, but then
I am still stuck with the differences in when I can access the caches.

The reason for the difference between the two is that in the case of
netdev_alloc_skb/frag the netdev_alloc_cache can only be accessed with
IRQs disabled, whereas in the napi_alloc_skb case we can access the
napi_alloc_cache at any point in the function. Either way I am going to
be stuck with differences because of the local_irq_save/restore that
must be called when accessing the page frag cache that doesn't exist in
the napi case.

- Alex

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