Messages in this thread | | | Subject | Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu() | From | Laurent Dufour <> | Date | Wed, 24 Apr 2019 09:57:05 +0200 |
| |
Le 23/04/2019 à 11:27, Peter Zijlstra a écrit : > On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote: >> This allows to search for a VMA structure without holding the mmap_sem. >> >> The search is repeated while the mm seqlock is changing and until we found >> a valid VMA. >> >> While under the RCU protection, a reference is taken on the VMA, so the >> caller must call put_vma() once it not more need the VMA structure. >> >> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a >> reference is taken to the VMA by calling get_vma(). >> >> When removing a VMA from the MM RB tree, the VMA is not release immediately >> but at the end of the RCU grace period through vm_rcu_put(). This ensures >> that the VMA remains allocated until the end the RCU grace period. >> >> Since the vm_file pointer, if valid, is released in put_vma(), there is no >> guarantee that the file pointer will be valid on the returned VMA. > > What I'm missing here, and in the previous patch introducing the > refcount (also see refcount_t), is _why_ we need the refcount thing at > all.
The need for the VMA's refcount is to ensure that the VMA will remain until the end of the SPF handler. This is a consequence of the use of RCU instead of SRCU to protect the RB tree.
I was not aware of the refcount_t type, it would be better here to avoid wrapping.
> My original plan was to use SRCU, which at the time was not complete > enough so I abused/hacked preemptible RCU, but that is no longer the > case, SRCU has all the required bits and pieces.
When I did test using SRCU it was involving a lot a scheduling to run the SRCU callback mechanism. In some workload the impact on the perfomance was significant [1].
I can't see this overhead using RCU.
> > Also; the initial motivation was prefaulting large VMAs and the > contention on mmap was killing things; but similarly, the contention on > the refcount (I did try that) killed things just the same.
Doing prefaulting should be doable, I'll try to think further about that.
Regarding the refcount, I should I missed something, this is an atomic counter, so there should not be contention on it but cache exclusivity, not ideal I agree but I can't see what else to use here.
> So I'm really sad to see the refcount return; and without any apparent > justification.
I'm not opposed to use another mechanism here, but SRCU didn't show good performance with some workload, and I can't see how to use RCU without a reference counter here. So please, advise.
Thanks, Laurent.
[1] https://lore.kernel.org/linux-mm/7ca80231-fe02-a3a7-84bc-ce81690ea051@intel.com/
| |