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SubjectRe: [PATCH v4 12/66] kernel/fork: Use maple tree for dup_mmap() during forking
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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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