On 3/11/21 9:59 AM, Mike Kravetz wrote:

On 3/11/21 4:17 AM, Michal Hocko wrote:

quoted

quoted

Yeah per cpu preempt counting shouldn't be noticeable but I have to
confess I haven't benchmarked it.

But all this seems moot now http://lkml.kernel.org/r/YEoA08n60+jzsnAl@hirez.programming.kicks-ass.net

The proper fix for free_huge_page independent of this series would
involve:

- Make hugetlb_lock and subpool lock irq safe
- Hand off freeing to a workque if the freeing could sleep

Today, the only time we can sleep in free_huge_page is for gigantic
pages allocated via cma.  I 'think' the concern about undesirable
user visible side effects in this case is minimal as freeing/allocating
1G pages is not something that is going to happen at a high frequency.
My thinking could be wrong?

Of more concern, is the introduction of this series.  If this feature
is enabled, then ALL free_huge_page requests must be sent to a workqueue.
Any ideas on how to address this?

Thinking about this more ...

A call to free_huge_page has two distinct outcomes
1) Page is freed back to the original allocator: buddy or cma
2) Page is put on hugetlb free list

We can only possibly sleep in the first case 1.  In addition, freeing a
page back to the original allocator involves these steps:
1) Removing page from hugetlb lists
2) Updating hugetlb counts: nr_hugepages, surplus
3) Updating page fields
4) Allocate vmemmap pages if needed as in this series
5) Calling free routine of original allocator

If hugetlb_lock is irq safe, we can perform the first 3 steps under that
lock without issue.  We would then use a workqueue to perform the last
two steps.  Since we are updating hugetlb user visible data under the
lock, there should be no delays.  Of course, giving those pages back to
the original allocator could still be delayed, and a user may notice
that.  Not sure if that would be acceptable?  I think Muchun had a
similar setup just for vmemmmap allocation in an early version of this
series.

This would also require changes to where accounting is done in
dissolve_free_huge_page and update_and_free_page as mentioned elsewhere.

P.S. We could further optimize to check for the possibility of sleeping
(cma or vmemmap) and only send to workqueue in those cases.
-- 
Mike Kravetz