Re: [PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare

Peter Xu
2022-12-09T16:49:54Z

On Fri, Dec 09, 2022 at 04:18:11PM +0100, David Hildenbrand wrote:
> On 09.12.22 15:39, Peter Xu wrote:
> > On Fri, Dec 09, 2022 at 11:24:55AM +0100, David Hildenbrand wrote:
> > > For such cases, it would be good to have any evidence that it really helps.
> >
> > I don't know much on the s390 path, but if a process has a large hugetlb
> > vma, even MADV_DONTNEED will be blocked for whatever long time if there's
> > another process or thread scanning pagemap for this vma.
> >
> > Would this justify a bit?
>
> I get your point. But that raises the question if we should voluntarily drop
> the VMA lock already in the caller every now and then on such large VMAs and
> maybe move even the cond_resched() into the common page walker, if you get
> what I mean?
>
> On a preemtible kernel you could reschedule just before you drop the lock
> and call cond_resched() ... hmm

On full preempt the old cond_resched() doesn't work anyway. IIUC most
distros now should be using dynamic preempt which makes voluntary mode by
default, which this change should help.

>
> No strong opinion here, it just looked a bit weird to optimize for a
> cond_resched() if we might just reschedule either way even without the
> cond_resched().

It's really not the core of the patchset. Let me drop it for now.

--
Peter Xu