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

[Michal Hocko]

On Wed 10-03-21 10:56:08, Mike Kravetz wrote:
> 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 haven't tried that but PREEMPT_COUNT should be selectable even without
any change to the preemption model (e.g. !PREEMPT).

-- 
Michal Hocko
SUSE Labs