Re: [PATCH bpf-next v2 0/5] execmem_alloc for BPF programs

From: Mike Rapoport <rppt@kernel.org>
To: Song Liu <song@kernel.org>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"hch@lst.de" <hch@lst.de>, "x86@kernel.org" <x86@kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"mcgrof@kernel.org" <mcgrof@kernel.org>,
	"Lu, Aaron" <aaron.lu@intel.com>
Subject: Re: [PATCH bpf-next v2 0/5] execmem_alloc for BPF programs
Date: Sun, 13 Nov 2022 12:42:50 +0200	[thread overview]
Message-ID: <Y3DKKivOwk+5rhNb@kernel.org> (raw)
In-Reply-To: <CAPhsuW5e8rBnu73DYkyc1L6gC-WBxjTZVwdFC_L12GVyzROR1w@mail.gmail.com>

On Wed, Nov 09, 2022 at 09:43:50AM -0800, Song Liu wrote:
> On Wed, Nov 9, 2022 at 3:18 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> [...]
> 
> > > >
> > > > The proposed execmem_alloc() looks to me very much tailored for x86
> > > > to be
> > > > used as a replacement for module_alloc(). Some architectures have
> > > > module_alloc() that is quite different from the default or x86
> > > > version, so
> > > > I'd expect at least some explanation how modules etc can use execmem_
> > > > APIs
> > > > without breaking !x86 architectures.
> > >
> > > I think this is fair, but I think we should ask ask ourselves - how
> > > much should we do in one step?
> >
> > I think that at least we need an evidence that execmem_alloc() etc can be
> > actually used by modules/ftrace/kprobes. Luis said that RFC v2 didn't work
> > for him at all, so having a core MM API for code allocation that only works
> > with BPF on x86 seems not right to me.
> 
> While using execmem_alloc() et. al. in module support is difficult, folks are
> making progress with it. For example, the prototype would be more difficult
> before CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC
> (introduced by Christophe).
> 
> We also have other users that we can onboard soon: BPF trampoline on
> x86_64, BPF jit and trampoline on arm64, and maybe also on powerpc and
> s390.

Caching of large pages won't make any difference on arm64 and powerpc
because they do not support splitting of the direct map, so the only
potential benefit there is a centralized handling of text loading and I'm
not convinced execmem_alloc() will get us there.

> > With execmem_alloc() as the first step I'm failing to see the large
> > picture. If we want to use it for modules, how will we allocate RO data?
> > with similar rodata_alloc() that uses yet another tree in vmalloc?
> > How the caching of large pages in vmalloc can be made useful for use cases
> > like secretmem and PKS?
> 
> If RO data causes problems with direct map fragmentation, we can use
> similar logic. I think we will need another tree in vmalloc for this case.
> Since the logic will be mostly identical, I personally don't think adding
> another tree is a big overhead.

Actually, it would be interesting to quantify memory savings/waste as the
result of using execmem_alloc()

> Thanks,
> Song

-- 
Sincerely yours,
Mike.