Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

[Mike Rapoport <rppt@kernel.org>]

On Mon, Apr 18, 2022 at 05:44:19PM -0700, Luis Chamberlain wrote:
> On Mon, Apr 18, 2022 at 01:06:36PM +0300, Mike Rapoport wrote:
> > Hi,
> > 
> > On Sat, Apr 16, 2022 at 10:26:08PM +0000, Song Liu wrote:
> > > > On Apr 16, 2022, at 1:30 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > > > 
> > > > Maybe I am missing something, but I really don't think this is ready
> > > > for prime-time. We should effectively disable it all, and have people
> > > > think through it a lot more.
> > > 
> > > This has been discussed on lwn.net: https://lwn.net/Articles/883454/. 
> > > AFAICT, the biggest concern is whether reserving minimal 2MB for BPF
> > > programs is a good trade-off for memory usage. This is again my fault
> > > not to state the motivation clearly: the primary gain comes from less 
> > > page table fragmentation and thus better iTLB efficiency. 
> > 
> > Reserving 2MB pages for BPF programs will indeed reduce the fragmentation,
> > but OTOH it will reduce memory utilization. If for large systems this may
> > not be an issue, on smaller machines trading off memory for iTLB
> > performance may be not that obvious.
> 
> So the current optimization at best should be a kconfig option?

Maybe not and it'll be fine on smaller systems, but from what I see the
bpf_prog_pack implementation didn't consider them.

And if we move the caches from BPF to vmalloc or page allocator that
would be much less of an issue.
 
> > I believe that "allocate huge page and split it to basic pages to hand out
> > to users" concept should be implemented at page allocator level and I
> > posted and RFC for this a while ago:
> > 
> > https://lore.kernel.org/all/20220127085608.306306-1-rppt@kernel.org/
> 
> Neat, so although eBPF is a big user, are there some use cases outside
> that immediately benefit?

Anything that uses set_memory APIs could benefit from this. Except eBPF and
other module_alloc() users, there is secretmem that also fractures the
direct map and actually that was my initial use case for these patches.

Another possible use-case can be protection of page tables with PKS:

https://lore.kernel.org/lkml/20210505003032.489164-1-rick.p.edgecombe@intel.com/

Vlastimil also mentioned that SEV-SNP could use such caching mechanism, but
I don't know the details.

>   LUis

-- 
Sincerely yours,
Mike.