Re: [PATCH] mm, mmap: remove vma_merge()

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Apr 01, 2024 at 09:26:24PM +0200, Vlastimil Babka wrote:
> With SLAB gone I was looking for what I could delete next, and then it
> dawned on me - it should be vma_merge(). It's simply an unmaintainable
> mess.
>
> The code is extremely hard to follow. There has been a number of recent
> attempts to made it more readable, by Liam, Lorenzo and myself. Almost
> every time this only resulted in subtle embarassing bugs [1] [2] [3] [4]
> [5]. For many of the bugs the only reliable testcase we have is the rpm
> process running in the Open Build Service. Which is worrying, as that
> clearly makes vma_merge() a potential supply chain attack vector for
> planting backdoors.
>
> Thus simply remove vma_merge() and all the related code, and adjust all
> callers to take the path as if merging was attempted and failed.
> The only downside is that there might now be more vmas and maple tree
> nodes due to no merging. To extensively validate that, I've booted the
> kernel with virtme-ng before and after the change and checked slabinfo
> for the number of active objects:
>
> before:
> maple_node          1159
> vm_area_struct      1538
>
> after:
> maple_node          1113
> vm_area_struct      1861
>
> Yeah there's more vma's but interestingly, less maple tree nodes. I
> believe it's worth it. The larger number of smaller vmas could even help
> with better parallelism thanks to per-vma locking. So the conclusion is
> obvious - we should have never tried merging vmas in the first place,
> hence the tags on this patch.
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d014cd7c1c35
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4c6759967826
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7e7757876f25
> [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1419430c8abb
> [5] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fc0c8f9089c2
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: <stable@vger.kenrel.org> # CVE mandatory
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Singed-off-by-one: Vlastimil Babka <vbabka@suse.cz>
> ---

[snip]

This is a beautiful patch and about as perfect a clean up as one could imagine
(I mean #thebestcodeisnocode right?), but as we have now eradicated VMA merging,
I can't be a hypocrite and advocate for patch merging.

As a result, sadly I must:

Hope-Liam-Nacks-It-By: Lorenzo Stoakes <lstoakes@gmail.com>