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

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Jul 06, 2022 at 09:47:32AM -0700, Alexei Starovoitov wrote:
> On Wed, Jul 6, 2022 at 8:59 AM Yafang Shao <laoar.shao@gmail.com> 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.
> >
> > 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.
> 
> Could you elaborate ?
> 
> > It also fixes a typo in the comment.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
> 
> Roman, do you agree with this change ?

Yes, removing __GFP_HIGH makes sense to me. I can imagine we might want
it for *some* bpf allocations, but applying it unconditionally looks wrong.

Thanks!