netdev - Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem

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Date: Thu, 18 Apr 2024 09:13:27 -0700
From: Song Liu <song@...nel.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>, Peter Zijlstra <peterz@...radead.org>, 
	linux-kernel@...r.kernel.org, Alexandre Ghiti <alexghiti@...osinc.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Bjorn Topel <bjorn@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	"David S. Miller" <davem@...emloft.net>, Dinh Nguyen <dinguyen@...nel.org>, 
	Donald Dutile <ddutile@...hat.com>, Eric Chanudet <echanude@...hat.com>, 
	Heiko Carstens <hca@...ux.ibm.com>, Helge Deller <deller@....de>, Huacai Chen <chenhuacai@...nel.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, Luis Chamberlain <mcgrof@...nel.org>, 
	Michael Ellerman <mpe@...erman.id.au>, Nadav Amit <nadav.amit@...il.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Puranjay Mohan <puranjay12@...il.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Russell King <linux@...linux.org.uk>, 
	Steven Rostedt <rostedt@...dmis.org>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
	Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>, bpf@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-mips@...r.kernel.org, linux-mm@...ck.org, linux-modules@...r.kernel.org, 
	linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	linux-s390@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev, 
	netdev@...r.kernel.org, sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()

On Thu, Apr 18, 2024 at 8:37 AM Mike Rapoport <rppt@...nel.org> wrote:
>
[...]
> >
> > Is +/- 2G enough for all realistic use cases? If so, I guess we don't
> > really need
> > EXECMEM_ANYWHERE below?
> >
> > > >
> > > > * I'm not sure about BPF's requirements; it seems happy doing the same as
> > > >   modules.
> > >
> > > BPF are happy with vmalloc().
> > >
> > > > So if we *must* use a common execmem allocator, what we'd reall want is our own
> > > > types, e.g.
> > > >
> > > >       EXECMEM_ANYWHERE
> > > >       EXECMEM_NOPLT
> > > >       EXECMEM_PREL32
> > > >
> > > > ... and then we use those in arch code to implement module_alloc() and friends.
> > >
> > > I'm looking at execmem_types more as definition of the consumers, maybe I
> > > should have named the enum execmem_consumer at the first place.
> >
> > I think looking at execmem_type from consumers' point of view adds
> > unnecessary complexity. IIUC, for most (if not all) archs, ftrace, kprobe,
> > and bpf (and maybe also module text) all have the same requirements.
> > Did I miss something?
>
> It's enough to have one architecture with different constrains for kprobes
> and bpf to warrant a type for each.
>

AFAICT, some of these constraints can be changed without too much work.

> Where do you see unnecessary complexity?
>
> > IOW, we have
> >
> > enum execmem_type {
> >         EXECMEM_DEFAULT,
> >         EXECMEM_TEXT,
> >         EXECMEM_KPROBES = EXECMEM_TEXT,
> >         EXECMEM_FTRACE = EXECMEM_TEXT,
> >         EXECMEM_BPF = EXECMEM_TEXT,      /* we may end up without
> > _KPROBE, _FTRACE, _BPF */
> >         EXECMEM_DATA,  /* rw */
> >         EXECMEM_RO_DATA,
> >         EXECMEM_RO_AFTER_INIT,
> >         EXECMEM_TYPE_MAX,
> > };
> >
> > Does this make sense?
>
> How do you suggest to deal with e.g. riscv that has separate address spaces
> for modules, kprobes and bpf?

IIUC, modules and bpf use the same address space on riscv, while kprobes use
vmalloc address. I haven't tried this yet, but I think we can let
kprobes use the
same space as modules and bpf, which is:

ffffffff00000000 |  -4     GB | ffffffff7fffffff |    2 GB | modules, BPF

Did I get this right?

Thanks,
Song