Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Mike Kravetz]

On 3/10/21 7:19 AM, Michal Hocko wrote:
> On Mon 08-03-21 18:28:02, Muchun Song wrote:
> [...]
>> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
>>  	/*
>>  	 * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
>>  	 */
>> -	if (!in_task()) {
>> +	if (in_atomic()) {
> 
> As I've said elsewhere in_atomic doesn't work for CONFIG_PREEMPT_COUNT=n.
> We need this change for other reasons and so it would be better to pull
> it out into a separate patch which also makes HUGETLB depend on
> PREEMPT_COUNT.

Yes, the issue of calling put_page for hugetlb pages from any context
still needs work.  IMO, that is outside the scope of this series.  We
already have code in this path which blocks/sleeps.

Making HUGETLB depend on PREEMPT_COUNT is too restrictive.  IIUC,
PREEMPT_COUNT will only be enabled if we enable:
PREEMPT "Preemptible Kernel (Low-Latency Desktop)"
PREEMPT_RT "Fully Preemptible Kernel (Real-Time)"
or, other 'debug' options.  These are not enabled in 'more common'
kernels.  Of course, we do not want to disable HUGETLB in common
configurations.

I'll put together a separate patch where we can discuss the merits of
making the change from !in_task to in_atomic, and what work remains in
this put_page area.
-- 
Mike Kravetz