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SubjectRe: [PATCH 4/8] HMM: Remove superflous RCU protection around radix tree lookup
On Mon, Mar 26, 2018 at 07:54:31AM -0700, Tejun Heo wrote:
> Hello, Andrew.
>
> Do you mind picking up the following patch? I can't find a good tree
> to route this through. The raw patch can be found at
>
> https://marc.info/?l=linux-mm&m=152105674112496&q=raw
>
> Thank you very much.

I am fine with which ever route there is low probability of conflict
when merging HMM through different tree.


>
> On Wed, Mar 14, 2018 at 12:45:11PM -0700, Tejun Heo wrote:
> > hmm_devmem_find() requires rcu_read_lock_held() but there's nothing
> > which actually uses the RCU protection. The only caller is
> > hmm_devmem_pages_create() which already grabs the mutex and does
> > superflous rcu_read_lock/unlock() around the function.
> >
> > This doesn't add anything and just adds to confusion. Remove the RCU
> > protection and open-code the radix tree lookup. If this needs to
> > become more sophisticated in the future, let's add them back when
> > necessary.
> >
> > Signed-off-by: Tejun Heo <tj@kernel.org>
> > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> > Cc: linux-mm@kvack.org
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > ---
> > Hello,
> >
> > Jérôme, how do you want to route this patch? If you prefer, I can
> > route it together with other patches.
> >
> > Thanks.
> >
> > mm/hmm.c | 12 ++----------
> > 1 file changed, 2 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 320545b98..d4627c5 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -845,13 +845,6 @@ static void hmm_devmem_release(struct device *dev, void *data)
> > hmm_devmem_radix_release(resource);
> > }
> >
> > -static struct hmm_devmem *hmm_devmem_find(resource_size_t phys)
> > -{
> > - WARN_ON_ONCE(!rcu_read_lock_held());
> > -
> > - return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT);
> > -}
> > -
> > static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
> > {
> > resource_size_t key, align_start, align_size, align_end;
> > @@ -892,9 +885,8 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
> > for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) {
> > struct hmm_devmem *dup;
> >
> > - rcu_read_lock();
> > - dup = hmm_devmem_find(key);
> > - rcu_read_unlock();
> > + dup = radix_tree_lookup(&hmm_devmem_radix,
> > + key >> PA_SECTION_SHIFT);
> > if (dup) {
> > dev_err(device, "%s: collides with mapping for %s\n",
> > __func__, dev_name(dup->device));
> > --
> > 2.9.5
> >
>
> --
> tejun

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