Re: [PATCH v2] mm: hugetlb: improve the handling of hugetlb allocation failure for freed or in-use hugetlb

[Baolin Wang]

On 2/5/2024 10:47 PM, Michal Hocko wrote:
> On Mon 05-02-24 20:50:51, Baolin Wang wrote:
> > When handling the freed hugetlb or in-use hugetlb, we should ignore the
> > failure of alloc_buddy_hugetlb_folio() to dissolve the old hugetlb successfully,
> > since we did not use the new allocated hugetlb in this 2 cases. Moreover,
> > moving the allocation into the free hugetlb handling branch.
>
> The changelog is a bit hard for me to understand. What about the
> following instead?
> alloc_and_dissolve_hugetlb_folio preallocates a new huge page before it
> takes hugetlb_lock. In 3 out of 4 cases the page is not really used and
> therefore the newly allocated page is just freed right away. This is
> wasteful and it might cause pre-mature failures in those cases.
>
> Address that by moving the allocation down to the only case (hugetlb
> page is really in the free pages pool). We need to drop hugetlb_lock
> to do so and therefore need to recheck the page state after regaining
> it.
>
> The patch is more of a cleanup than an actual fix to an existing
> problem. There are no known reports about pre-mature failures.

Looks better. Thanks.

> > @@ -3075,6 +3063,24 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
> >   		cond_resched();
> >   		goto retry;
> >   	} else {
> > +		if (!new_folio) {
> > +			spin_unlock_irq(&hugetlb_lock);
> > +			/*
> > +			 * Before dissolving the free hugetlb, we need to allocate
> > +			 * a new one for the pool to remain stable.  Here, we
> > +			 * allocate the folio and 'prep' it by doing everything
> > +			 * but actually updating counters and adding to the pool.
> > +			 * This simplifies and let us do most of the processing
> > +			 * under the lock.
> > +			 */
>
> This comment is not really needed anymore IMHO.

Acked.

> > +			new_folio = alloc_buddy_hugetlb_folio(h, gfp_mask, nid,
> > +							      NULL, NULL);
> > +			if (!new_folio)
> > +				return -ENOMEM;
> > +			__prep_new_hugetlb_folio(h, new_folio);
> > +			goto retry;
> > +		}
> > +
> >   		/*
> >   		 * Ok, old_folio is still a genuine free hugepage. Remove it from
> >   		 * the freelist and decrease the counters. These will be