Re: [PATCH v4 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Oct 09, 2024 at 12:17:12AM -0700, Namhyung Kim wrote:
> On Mon, Oct 07, 2024 at 02:57:08PM +0200, Vlastimil Babka wrote:
> > On 10/4/24 11:25 PM, Roman Gushchin wrote:
> > > On Fri, Oct 04, 2024 at 01:10:58PM -0700, Song Liu wrote:
> > >> On Wed, Oct 2, 2024 at 11:10 AM Namhyung Kim <namhyung@kernel.org> wrote:
> > >>>
> > >>> The bpf_get_kmem_cache() is to get a slab cache information from a
> > >>> virtual address like virt_to_cache().  If the address is a pointer
> > >>> to a slab object, it'd return a valid kmem_cache pointer, otherwise
> > >>> NULL is returned.
> > >>>
> > >>> It doesn't grab a reference count of the kmem_cache so the caller is
> > >>> responsible to manage the access.  The intended use case for now is to
> > >>> symbolize locks in slab objects from the lock contention tracepoints.
> > >>>
> > >>> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> > >>> Acked-by: Roman Gushchin <roman.gushchin@linux.dev> (mm/*)
> > >>> Acked-by: Vlastimil Babka <vbabka@suse.cz> #mm/slab
> > >>> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > 
> > 
> > So IIRC from our discussions with Namhyung and Arnaldo at LSF/MM I
> > thought the perf use case was:
> > 
> > - at the beginning it iterates the kmem caches and stores anything of
> > possible interest in bpf maps or somewhere - hence we have the iterator
> > - during profiling, from object it gets to a cache, but doesn't need to
> > access the cache - just store the kmem_cache address in the perf record
> > - after profiling itself, use the information in the maps from the first
> > step together with cache pointers from the second step to calculate
> > whatever is necessary
> 
> Correct.
> 
> > 
> > So at no point it should be necessary to take refcount to a kmem_cache?
> > 
> > But maybe "bpf_get_kmem_cache()" is implemented here as too generic
> > given the above use case and it should be implemented in a way that the
> > pointer it returns cannot be used to access anything (which could be
> > unsafe), but only as a bpf map key - so it should return e.g. an
> > unsigned long instead?
> 
> Yep, this should work for my use case.  Maybe we don't need the
> iterator when bpf_get_kmem_cache() kfunc returns the valid pointer as
> we can get the necessary info at the moment.  But I think it'd be less
> efficient as more work need to be done at the event (lock contention).
> It'd better setting up necessary info in a map before monitoring (using
> the iterator), and just looking up the map with the kfunc while
> monitoring the lock contention.

Maybe it's still better to return a non-refcounted pointer for future
use.  I'll leave it for v5.

Thanks,
Namhyung