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28, 2025 at 12:02=E2=80=AFPM Liam R. Howlett > wrote: > > > > A previously used maple state may not be in the correct state for the > > preallocation to correctly calculate the number of nodes necessary for = the > > configured store operation. > > > > The user visible effect of which is warning that there are no nodes > > allocated followed by a crash when there is a null pointer dereference > > shortly after. > > > > The NULL pointer dereference has been reported to happen when a vma > > iterator is used to preallocate after walking to a leaf node but then > > requesting to preallocate for a store across node boundaries (in v6.6. > > of the kernel). The altered range means that there may not been enough > > nodes as the maple state has been incorrectly configured. A critical > > step is that the vma iterator then detects the misconfigured maple stat= e > > and resets, thus ensuring the tree is not corrupted - but ultimately > > causes a failure when there are no nodes left. > > > > Detect a misconfigured maple state in the mas_preallocate() code by > > examining the current location and planned write to ensure a reset is > > done if required. The performance impacts are minimal and within the > > noise in initial testing. >=20 > With this fix applied I can still see the issue using Hailong's > reproducers, both the userspace one that he shared over the weekend > and the one posted at > https://lore.kernel.org/all/1652f7eb-a51b-4fee-8058-c73af63bacd1@oppo.com= / Thanks. The test patch also has a few issues which would affect the allocations (such as not setting the rcu flag on the tree). Besides that, there are a few bugs here. One is the incorrect calculation due to the moving vma iterator - which my patch doesn't quite address as I've come up with another scenario, and test case. I suspect the rule of the vma iterator pointing at the 'correct slot' (documented but not enforced) saved us a lot of issues here. I'm undecided if it's worth detecting this being false and resetting, like in this patch. The other is to do with MA_STATE_PREALLOC, which will stop preallocations in bulk insert mode. If it's left set from the first mas_preallocate() call, then we won't allocate more nodes and simply return. That is, the bulk allocation is causing issues with chaining mas_preallocate() calls. There is another issue with this flag not always being set, and so my testing passed... With the way this month has been going, I expect more fun yet. I'll send out another version of this RFC with two (or, I guess more..) patches soon. Annoyingly, not all of these bugs exist in upstream - and so the vma test code is less useful than it will be in the future. I have recreated the scenario using the test code, but it didn't create the same issues. Thanks, Liam