| Date | Thu, 16 Dec 2021 12:09:13 +0100 | Subject | Re: [PATCH v4 12/66] kernel/fork: Use maple tree for dup_mmap() during forking | From | Vlastimil Babka <> |
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On 12/1/21 15:29, Liam Howlett wrote: > From: "Liam R. Howlett" <Liam.Howlett@Oracle.com> > > The maple tree was already tracking VMAs in this function by an earlier > commit, but the rbtree iterator was being used to iterate the list. > Change the iterator to use a maple tree native iterator and switch to > the maple tree advanced API to avoid multiple walks of the tree during > insert operations. Unexport the now-unused vma_store() function. > > We track whether we need to free the VMAs and tree nodes through RCU > (ie whether there have been multiple threads that can see the mm_struct > simultaneously; by pthread(), ptrace() or looking at /proc/$pid/maps). > This setting is sticky because it's too tricky to decide when it's safe > to exit RCU mode.
I don't immediately see why enabling the RCU tracking in mmget is part of the dup_mmap() change?
> For performance reasons we bulk allocate the maple tree nodes. The node > calculations are done internally to the tree and use the VMA count and > assume the worst-case node requirements. The VM_DONT_COPY flag does > not allow for the most efficient copy method of the tree and so a bulk > loading algorithm is used. > > Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
> static inline bool mmget_not_zero(struct mm_struct *mm) > { > + /* > + * There is a race below during task tear down that can cause the maple
What does 'below' refer to here?
> + * tree to enter rcu mode with only a single user. If this race > + * happens, the result would be that the maple tree nodes would remain > + * active for an extra RCU read cycle. > + */ > + if (!mt_in_rcu(&mm->mm_mt)) > + mm_set_in_rcu(mm); > return atomic_inc_not_zero(&mm->mm_users); > } > > diff --git a/kernel/fork.c b/kernel/fork.c > index cc9bb95c7678..c9f8465d8ae2 100644
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