Re: [PATCH bpf-next v2 1/2] bpf: Make non-preallocated allocation low priority

[Shakeel Butt <shakeelb@google.com>]

On Wed, Jul 06, 2022 at 03:58:47PM +0000, Yafang Shao wrote:
> GFP_ATOMIC doesn't cooperate well with memcg pressure so far, especially
> if we allocate too much GFP_ATOMIC memory. For example, when we set the
> memcg limit to limit a non-preallocated bpf memory, the GFP_ATOMIC can
> easily break the memcg limit by force charge. So it is very dangerous to
> use GFP_ATOMIC in non-preallocated case. One way to make it safe is to
> remove __GFP_HIGH from GFP_ATOMIC, IOW, use (__GFP_ATOMIC |
> __GFP_KSWAPD_RECLAIM) instead, then it will be limited if we allocate
> too much memory.

Please use GFP_NOWAIT instead of (__GFP_ATOMIC | __GFP_KSWAPD_RECLAIM).
There is already a plan to completely remove __GFP_ATOMIC and mm-tree
already have a patch for that.

> 
> We introduced BPF_F_NO_PREALLOC is because full map pre-allocation is
> too memory expensive for some cases. That means removing __GFP_HIGH
> doesn't break the rule of BPF_F_NO_PREALLOC, but has the same goal with
> it-avoiding issues caused by too much memory. So let's remove it.
> 
> The force charge of GFP_ATOMIC was introduced in
> commit 869712fd3de5 ("mm: memcontrol: fix network errors from failing
> __GFP_ATOMIC charges") by checking __GFP_ATOMIC, then got improved in
> commit 1461e8c2b6af ("memcg: unify force charging conditions") by
> checking __GFP_HIGH (that is no problem because both __GFP_HIGH and
> __GFP_ATOMIC are set in GFP_AOMIC). So, if we want to fix it in memcg,
> we have to carefully verify all the callsites. Now that we can fix it in
> BPF, we'd better not modify the memcg code.
> 
> This fix can also apply to other run-time allocations, for example, the
> allocation in lpm trie, local storage and devmap. So let fix it
> consistently over the bpf code
> 
> __GFP_KSWAPD_RECLAIM doesn't cooperate well with memcg pressure neither
> currently. But the memcg code can be improved to make
> __GFP_KSWAPD_RECLAIM work well under memcg pressure if desired.
> 

IMO there is no need to give all this detail and background on
GFP_ATOMIC and __GFP_KSWAPD_RECLAIM. Just say kernel allows GFP_ATOMIC
allocations to exceed memcg limits which we don't want in this case. So,
replace with GFP_NOWAIT which obey memcg limits. Both of these flags
tell kernel that the caller can not sleep.