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

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Date: Mon, 5 Feb 2024 15:47:31 +0100
From: Michal Hocko <mhocko@...e.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, muchun.song@...ux.dev, osalvador@...e.de,
	david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: hugetlb: improve the handling of hugetlb
 allocation failure for freed or in-use hugetlb

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.

[...]

> @@ -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.

> +			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

-- 
Michal Hocko
SUSE Labs