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SubjectRe: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()
On Thu, Apr 18, 2024 at 11:56 PM Mike Rapoport <rppt@kernel.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@kernel.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@kernel.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.

Did I miss something?

Thanks,
Song

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