On Thu, Jun 30, 2022 at 11:47:00PM +0200, Daniel Borkmann wrote: > Hi Yafang, > > On 6/29/22 5:48 PM, 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. > > > > 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. > > > > __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. > > Ok, but could you also explain in commit desc why it's a specific problem > to BPF hashtab? > > Afaik, there is plenty of other code using GFP_ATOMIC | __GFP_NOWARN outside > of BPF e.g. under net/, so it's a generic memcg problem? I'd be careful here and not change it all together. __GFP_NOWARN might be used to suppress warnings which otherwise would be too verbose and disruptive (especially if we talk about /net allocations in conjunction with netconsole) and might not mean a low/lower priority. > Why are lpm trie and local storage map for BPF not affected (at least I don't > see them covered in the patch)? Yes, it would be nice to fix this consistently over the bpf code. Yafang, would you mind to fix it too? Thanks! Roman
