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kNPnGdtwaVs= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 11, 2025 at 03:03:36PM +0200, David Hildenbrand wrote: > On 11.08.25 14:14, Lorenzo Stoakes wrote: > > On Mon, Aug 11, 2025 at 03:13:14PM +0530, Charan Teja Kalla wrote: > > > Thanks David, for the reply!! > > > On 8/8/2025 5:34 PM, David Hildenbrand wrote: > > > > >         if (mpnt) { > > > > >             mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1); > > > > >             mas_store(&vmi.mas, XA_ZERO_ENTRY); > > > > >             /* Avoid OOM iterating a broken tree */ > > > > >             set_bit(MMF_OOM_SKIP, &mm->flags); > > > > >         } > > > > >         /* > > > > >          * The mm_struct is going to exit, but the locks will be dropped > > > > >          * first.  Set the mm_struct as unstable is advisable as it is > > > > >          * not fully initialised. > > > > >          */ > > > > >         set_bit(MMF_UNSTABLE, &mm->flags); > > > > >     } > > > > > > > > > > Shouldn't we just remove anything from the tree here that was not copied > > > > > immediately? > > > > > > > > Another fix would be to just check MMF_UNSTABLE in unuse_mm(). But > > > > having these MMF_UNSTABLE checks all over the place feels a bit like > > > > whack-a-mole. > > > > > > > Seems MMF_UNSTABLE is the expectation per the commit, > > > 64c37e134b12("kernel: be more careful about dup_mmap() failures and > > > uprobe registering"). Excerpt(s) from the commit message: > > > > This really is whack-a-mole yeah. > > > > > > > > This patch sets the MMF_OOM_SKIP to avoid the iteration of the vmas on > > > the oom side (even though this is extremely unlikely to be selected as > > > an oom victim in the race window), and __sets MMF_UNSTABLE to avoid > > > other potential users from using a partially initialised mm_struct. > > > > > > > But... maybe this is better for the _hotfix_ version as a nicer way of > > doing this. > > I would prefer using MMF_UNSTABLE as a hotfix. Yeah, I think MMF_UNSTABLE is probably the way go, this is what I was trying to say :P > > > > > > When registering vmas for uprobe, skip the vmas in an mm that is marked > > > unstable. Modifying a vma in an unstable mm may cause issues if the mm > > > isn't fully initialised.__ > > > > > > > Is there anything preventing us from just leaving a proper tree that > > > > reflects reality in place before we drop the write lock? > > > > > > When you mean proper tree, is this about the your previous question? -- > > > Shouldn't we just remove anything from the tree here that was not copied > > > immediately? > > > > Commit d24062914837 (" fork: use __mt_dup() to duplicate maple tree in > > dup_mmap()") did this for efficiency, so it'd be a regression to do this. > > We're talking about the case where fork *fails*. That cannot possibly be > relevant for performance, can it? :) I think it optimises the overall operation, but as a product of that, has to handle this edge case, and that necessitated this rather horrble stuff. Obviously we don't need to optimise a 'we are about to die' case :) See https://lore.kernel.org/linux-mm/20231016032226.59199-1-zhangpeng.00@bytedance.com/ for details. Cheers, Lorenzo