netdev - Re: [PATCH bpf 0/4] introduce HAVE_ARCH_HUGE_VMALLOC_FLAG for bpf_prog

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Date:   Thu, 31 Mar 2022 00:46:47 +0000
From:   Song Liu <songliubraving@...com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "song@...nel.org" <song@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "pmenzel@...gen.mpg.de" <pmenzel@...gen.mpg.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "ast@...nel.org" <ast@...nel.org>
Subject: Re: [PATCH bpf 0/4] introduce HAVE_ARCH_HUGE_VMALLOC_FLAG for
 bpf_prog_pack



> On Mar 30, 2022, at 5:04 PM, Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote:
> 
> On Wed, 2022-03-30 at 15:56 -0700, Song Liu wrote:
>> [1] 
>> https://lore.kernel.org/lkml/5bd16e2c06a2df357400556c6ae01bb5d3c5c32a.camel@intel.com/
> 
> The issues I brought up around VM_FLUSH_RESET_PERMS are not fixed in
> this series. And I think the solution I proposed is kind of wonky with
> respect to hibernate. So I think maybe hibernate should be fixed to not
> impose restrictions on the direct map, so the wonkiness is not needed.
> But then this "fixes" series becomes quite extensive.
> 
> I wonder, why not just push the patch 1 here, then re-enable this thing
> when it is all properly fixed up. It looked like your code could handle
> the allocation not actually getting large pages.

Only shipping patch 1 should eliminate the issues. But that will also
reduce the benefit in iTLB efficiency (I don't know by how much yet.)

> 
> Another solution that would keep large pages but still need fixing up
> later: Just don't use VM_FLUSH_RESET_PERMS for now. Call
> set_memory_nx() and then set_memory_rw() on the module space address
> before vfree(). This will clean up everything that's needed with
> respect to direct map permissions. Have vmalloc warn if is sees
> VM_FLUSH_RESET_PERMS and huge pages together.

Do you mean we should remove set_vm_flush_reset_perms() from 
alloc_new_pack() and do set_memory_nx() and set_memory_rw() before
we call vfree() in bpf_prog_pack_free()? If this works, I would prefer
we go with this way. 

Thanks,
Song