On Tue, Oct 17, 2017 at 04:55:52PM +0200, Daniel Borkmann wrote: > Add an option for pcpu_alloc() to support __GFP_NOWARN flag. > Currently, we always throw a warning when size or alignment > is unsupported (and also dump stack on failed allocation > requests). The warning itself is harmless since we return > NULL anyway for any failed request, which callers are > required to handle anyway. However, it becomes harmful when > panic_on_warn is set. > > The rationale for the WARN() in pcpu_alloc() is that it can > be tracked when larger than supported allocation requests are > made such that allocations limits can be tweaked if warranted. > This makes sense for in-kernel users, however, there are users > of pcpu allocator where allocation size is derived from user > space requests, e.g. when creating BPF maps. In these cases, > the requests should fail gracefully without throwing a splat. > > The current work-around was to check allocation size against > the upper limit of PCPU_MIN_UNIT_SIZE from call-sites for > bailing out prior to a call to pcpu_alloc() in order to > avoid throwing the WARN(). This is bad in multiple ways since > PCPU_MIN_UNIT_SIZE is an implementation detail, and having > the checks on call-sites only complicates the code for no > good reason. Thus, lets fix it generically by supporting the > __GFP_NOWARN flag that users can then use with calling the > __alloc_percpu_gfp() helper instead. > > Signed-off-by: Daniel Borkmann <dan...@iogearbox.net> > Cc: Tejun Heo <t...@kernel.org> > Cc: Mark Rutland <mark.rutl...@arm.com>
The approach looks great to me. We've been doing this dance around allocator warning for long time. It's really not a job of bpf code to guess into valid parameters of pcpu alloc. Adding support for __GFP_NOWARN and using it in bpf is much cleaner fix that avoids layering violations. Acked-by: Alexei Starovoitov <a...@kernel.org>
