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@xxxxxxxxxxxxxxx> # CVE mandatory > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx> > Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> > Singed-off-by-one: Vlastimil Babka <vbabka@xxxxxxx> > --- [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@xxxxxxxxx>
