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Date: Thu, 18 Apr 2024 14:41:23 +0200
From: Oscar Salvador <osalvador@suse.de>
To: Miaohe Lin <linmiaohe@huawei.com>
Cc: akpm@linux-foundation.org, muchun.song@linux.dev, david@redhat.com,
	vbabka@suse.cz, willy@infradead.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm/hugetlb: fix DEBUG_LOCKS_WARN_ON(1) when
 dissolve_free_hugetlb_folio()
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References: <20240418022000.3524229-1-linmiaohe@huawei.com>
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In-Reply-To: <a852eb0c-f64e-f1d7-d685-a4e81a6416c6@huawei.com>

On Thu, Apr 18, 2024 at 04:00:42PM +0800, Miaohe Lin wrote:
> On 2024/4/18 12:05, Oscar Salvador wrote:
> > On Thu, Apr 18, 2024 at 10:19:59AM +0800, Miaohe Lin wrote:
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index 26ab9dfc7d63..1da9a14a5513 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -1788,7 +1788,8 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
> >>  		destroy_compound_gigantic_folio(folio, huge_page_order(h));
> >>  		free_gigantic_folio(folio, huge_page_order(h));
> >>  	} else {
> >> -		INIT_LIST_HEAD(&folio->_deferred_list);
> >> +		if (!folio_test_hugetlb(folio))
> >> +			INIT_LIST_HEAD(&folio->_deferred_list);
> > 
> > Ok, it took me a bit to figure this out.
> > 
> > So we basically init __deferred_list when we know that
> > folio_put will not end up calling free_huge_folio
> > because a previous call to remove_hugetlb_folio has already cleared the
> > bit.
> > 
> > Maybe Matthew thought that any folio ending here would not end up in
> > free_huge_folio (which is the one fiddling subpool).
> > 
> > I mean, fix looks good because if hugetlb flag is cleared,
> > destroy_large_folio will go straight to free_the_page, but the
> > whole thing is a bit subtle.
> 
> AFAICS, this is the most straightforward way to fix the issue. Do you have any suggestions
> on how to fix this in a more graceful way?

Not from the top of my head.
Anyway, I have been thinking for a while that this code needs some love,
so I will check how this can be untangled.


-- 
Oscar Salvador
SUSE Labs