linux-kernel - Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE

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Date:   Fri, 15 Apr 2022 18:43:08 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Song Liu <song@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     bpf <bpf@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Christoph Hellwig <hch@...radead.org>, imbrenda@...ux.ibm.com
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

On Fri, Apr 15, 2022 at 06:42:27PM -0700, Luis Chamberlain wrote:
> On Fri, Apr 15, 2022 at 06:34:16PM -0700, Song Liu wrote:
> > On Fri, Apr 15, 2022 at 12:05 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
> > >
> > > On Fri, Apr 15, 2022 at 09:44:09AM -0700, Song Liu wrote:
> > > > Changes v3 => v4:
> > > > 1. Fix __weak module_alloc_huge; remove unused vmalloc_huge; rename
> > > >    __vmalloc_huge => vmalloc_huge. (Christoph Hellwig)
> > > > 2. Use vzalloc (as it was before vmalloc_no_huge) and clean up comments in
> > > >    kvm_s390_pv_alloc_vm.
> > > >
> > > > Changes v2 => v3:
> > > > 1. Use __vmalloc_huge in alloc_large_system_hash.
> > > > 2. Use EXPORT_SYMBOL_GPL for new functions. (Christoph Hellwig)
> > > > 3. Add more description about the issues and changes.(Christoph Hellwig,
> > > >    Rick Edgecombe).
> > > >
> > > > Changes v1 => v2:
> > > > 1. Add vmalloc_huge(). (Christoph Hellwig)
> > > > 2. Add module_alloc_huge(). (Christoph Hellwig)
> > > > 3. Add Fixes tag and Link tag. (Thorsten Leemhuis)
> > > >
> > > > Enabling HAVE_ARCH_HUGE_VMALLOC on x86_64 and use it for bpf_prog_pack has
> > > > caused some issues [1], as many users of vmalloc are not yet ready to
> > > > handle huge pages. To enable a more smooth transition to use huge page
> > > > backed vmalloc memory, this set replaces VM_NO_HUGE_VMAP flag with an new
> > > > opt-in flag, VM_ALLOW_HUGE_VMAP. More discussions about this topic can be
> > > > found at [2].
> > > >
> > > > Patch 1 removes VM_NO_HUGE_VMAP and adds VM_ALLOW_HUGE_VMAP.
> > > > Patch 2 uses VM_ALLOW_HUGE_VMAP in bpf_prog_pack.
> > > >
> > > > [1] https://lore.kernel.org/lkml/20220204185742.271030-1-song@kernel.org/
> > > > [2] https://lore.kernel.org/linux-mm/20220330225642.1163897-1-song@kernel.org/
> > >
> > > Looks good except for that I think this should just wait for v5.19. The
> > > fixes are so large I can't see why this needs to be rushed in other than
> > > the first assumptions of the optimizations had some flaws addressed here.
> > 
> > We need these changes to fix issues like [3]. Note that there might
> > still be some
> > undiscovered issues with huge page backed vmalloc memory on powerpc, which
> > had HAVE_ARCH_HUGE_VMALLOC enabled since the 5.15 kernel. As we
> > agreed, the new opt-in flag is a safer approach here. We probably should have
> > 1/4 and 2/4 back ported to stable. Therefore, I think shipping this
> > set now would
> > give us a more reliable 5.18 release.
> > 
> > Does this make sense?
> 
> Yes absolutely, but that sounds like that optimization should just be
> reverted completely from v5.18 isntead then no? Or if one can skip the
> optimizations for v5.18 with a small patch, and then enable for v5.19.

In any case it is up to Linus, I already chimed in with my opinion.

  Luis