linux-kernel - Re: [PATCH] mm, mmap: remove vma

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Date: Mon, 01 Apr 2024 22:08:31 +0200
From: Oleksandr Natalenko <oleksandr@...alenko.name>
To: linux-mm@...ck.org, Vlastimil Babka <vbabka@...e.cz>
Cc: linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
 Vlastimil Babka <vbabka@...e.cz>, stable@...r.kenrel.org,
 Lorenzo Stoakes <lstoakes@...il.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH] mm, mmap: remove vma_merge()

Hello.

On pondělí 1. dubna 2024 21:26:24, CEST 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@...r.kenrel.org> # CVE mandatory
> Cc: Lorenzo Stoakes <lstoakes@...il.com>
> Cc: Liam R. Howlett <Liam.Howlett@...cle.com>
> Singed-off-by-one: Vlastimil Babka <vbabka@...e.cz>

Nyacked-by: Oleksandr Natalenko <oleksandr@...alenko.name>

TYVM!

> ---
>  mm/internal.h |   7 -
>  mm/mmap.c     | 368 +++-----------------------------------------------
>  mm/mremap.c   |  60 --------
>  3 files changed, 17 insertions(+), 418 deletions(-)

/* SNIP */

-- 
Oleksandr Natalenko (post-factum)
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