Re: [PATCH v3 2/2] mm: migrate: fix barriers around tlb_flush_pending

[Nadav Amit <nadav.amit@gmail.com>]

Mel Gorman <mgorman@suse.de> wrote:

> On Thu, Jul 27, 2017 at 04:40:15AM -0700, Nadav Amit wrote:
>> Reading tlb_flush_pending while the page-table lock is taken does not
>> require a barrier, since the lock/unlock already acts as a barrier.
>> Removing the barrier in mm_tlb_flush_pending() to address this issue.
>> 
>> However, migrate_misplaced_transhuge_page() calls mm_tlb_flush_pending()
>> while the page-table lock is already released, which may present a
>> problem on architectures with weak memory model (PPC). To deal with this
>> case, a new parameter is added to mm_tlb_flush_pending() to indicate
>> if it is read without the page-table lock taken, and calling
>> smp_mb__after_unlock_lock() in this case.
>> 
>> Signed-off-by: Nadav Amit <namit@vmware.com>
> 
> Conditional locking based on function arguements are often considered
> extremely hazardous. Conditional barriers are even more troublesome because
> it's simply too easy to get wrong.
> 
> Revert b0943d61b8fa420180f92f64ef67662b4f6cc493 instead of this patch. It's
> not a clean revert but conflicts are due to comment changes. It moves
> the check back under the PTL and the impact is marginal given that
> it a spurious TLB flush will only occur when potentially racing with
> change_prot_range. Since that commit went in, a lot of changes have happened
> that alter the scan rate of automatic NUMA balancing so it shouldn't be a
> serious issue. It's certainly a nicer option than using conditional barriers.

Ok. Initially, I added a memory barrier only in
migrate_misplaced_transhuge_page(), and included a detailed comment about it
- I still think it is better. But since you feel confident the impact will
be relatively small, I will do the revert.


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>