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Date:   Wed, 13 Jul 2022 22:16:36 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Song Liu <song@kernel.org>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-modules@vger.kernel.org, mcgrof@kernel.org,
        rostedt@goodmis.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, mhiramat@kernel.org, naveen.n.rao@linux.ibm.com,
        davem@davemloft.net, anil.s.keshavamurthy@intel.com,
        keescook@chromium.org, hch@infradead.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 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?