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

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Message-ID: <ZiKjmaDgz_56ovbv@kernel.org>
Date: Fri, 19 Apr 2024 20:02:17 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Song Liu <song@...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 Fri, Apr 19, 2024 at 08:54:40AM -0700, Song Liu wrote:
> On Thu, Apr 18, 2024 at 11:56 PM Mike Rapoport <rppt@...nel.org> wrote:
> >
> > On Thu, Apr 18, 2024 at 02:01:22PM -0700, Song Liu wrote:
> > > On Thu, Apr 18, 2024 at 10:54 AM Mike Rapoport <rppt@...nel.org> wrote:
> > > >
> > > > On Thu, Apr 18, 2024 at 09:13:27AM -0700, Song Liu wrote:
> > > > > On Thu, Apr 18, 2024 at 8:37 AM Mike Rapoport <rppt@...nel.org> wrote:
> > > > > > > >
> > > > > > > > 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.
> > > >
> > > > But why?
> > > > I honestly don't understand what are you trying to optimize here. A few
> > > > lines of initialization in execmem_info?
> > >
> > > IIUC, having separate EXECMEM_BPF and EXECMEM_KPROBE makes it
> > > harder for bpf and kprobe to share the same ROX page. In many use cases,
> > > a 2MiB page (assuming x86_64) is enough for all BPF, kprobe, ftrace, and
> > > module text. It is not efficient if we have to allocate separate pages for each
> > > of these use cases. If this is not a problem, the current approach works.
> >
> > The caching of large ROX pages does not need to be per type.
> >
> > In the POC I've posted for caching of large ROX pages on x86 [1], the cache is
> > global and to make kprobes and bpf use it it's enough to set a flag in
> > execmem_info.
> >
> > [1] https://lore.kernel.org/all/20240411160526.2093408-1-rppt@kernel.org
> 
> For the ROX to work, we need different users (module text, kprobe, etc.) to have
> the same execmem_range. From [1]:
> 
> static void *execmem_cache_alloc(struct execmem_range *range, size_t size)
> {
> ...
>        p = __execmem_cache_alloc(size);
>        if (p)
>                return p;
>       err = execmem_cache_populate(range, size);
> ...
> }
> 
> We are calling __execmem_cache_alloc() without range. For this to work,
> we can only call execmem_cache_alloc() with one execmem_range.

Actually, on x86 this will "just work" because everything shares the same
address space :)

The 2M pages in the cache will be in the modules space, so
__execmem_cache_alloc() will always return memory from that address space.

For other architectures this indeed needs to be fixed with passing the
range to __execmem_cache_alloc() and limiting search in the cache for that
range.
 
> Did I miss something?
> 
> Thanks,
> Song

-- 
Sincerely yours,
Mike.