On Thu, Jul 7, 2022 at 8:07 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > 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. > After reading the discussion[1], it looks good to me to use GFP_NOWAIT instead. I will update it. [1]. https://lore.kernel.org/linux-mm/163712397076.13692.4727608274002939094@xxxxxxxxxxxxxxxxxxxxx/ > > > > 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. > Sure, thanks. -- Regards Yafang
