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Date:   Thu, 14 Jul 2022 09:26:22 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Song Liu <song@kernel.org>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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        bp@alien8.de, mhiramat@kernel.org, naveen.n.rao@linux.ibm.com,
        davem@davemloft.net, anil.s.keshavamurthy@intel.com,
        keescook@chromium.org, dave@stgolabs.net, daniel@iogearbox.net,
        kernel-team@fb.com, x86@kernel.org, dave.hansen@linux.intel.com,
        rick.p.edgecombe@intel.com, akpm@linux-foundation.org
Subject: Re: [PATCH bpf-next 1/3] mm/vmalloc: introduce vmalloc_exec which
 allocates RO+X memory
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On Wed, Jul 13, 2022 at 10:16:36PM -0700, Christoph Hellwig wrote:
> On Wed, Jul 13, 2022 at 12:20:09PM +0200, Peter Zijlstra wrote:
> > Start by adding VM_TOPDOWN_VMAP, which instead of returning the lowest
> > (leftmost) vmap_area that fits, picks the higests (rightmost).
> > 
> > Then add module_alloc_data() that uses VM_TOPDOWN_VMAP and make
> > ARCH_WANTS_MODULE_DATA_IN_VMALLOC use that instead of vmalloc (with a
> > weak function doing the vmalloc).
> > 
> > This gets you bottom of module range is RO+X only, top is shattered
> > between different !X types.
> > 
> > Then track the boundary between X and !X and ensure module_alloc_data()
> > and module_alloc() never cross over and stay strictly separated.
> > 
> > Then change all module_alloc() users to expect RO+X memory, instead of
> > RW.
> > 
> > Then make sure any extention of the X range is 2M aligned.
> > 
> > And presto, *everybody* always uses 2M TLB for text, modules, bpf,
> > ftrace, the lot and nobody is tracking chunks.
> > 
> > Maybe migration can be eased by instead providing module_alloc_text()
> > and ARCH_WANTS_MODULE_ALLOC_TEXT.
> 
> This all looks pretty sensible.  How are we going to do the initial
> write to the executable memory, though?

With something like text_poke_memcpy(). I suppose that the proposed
ARCH_WANTS_MODULE_ALLOC_TEXT needs to imply availability of that too.

If the 4K copy thing ends up being a bottleneck we can easily extend
that to have a 2M option as well.