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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 03:13:14PM +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. > 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. See https://lore.kernel.org/linux-mm/20231016032226.59199-1-zhangpeng.00@bytedance.com/ > > Anyway, would request Liam/Lorenzo to comment on this. Hi! :) Really want Liam's input on this too, as he looked at this previously and was, I believe, at least potentially going to take a look around this whole logic. Fork is sadly an area in which things become horrible quick... Cheers, Lorenzo