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Subject: Re: [PATCH 1/2] mm/hugetlb: fix DEBUG_LOCKS_WARN_ON(1) when
 dissolve_free_hugetlb_folio()
To: Oscar Salvador <osalvador@suse.de>
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>
References: <20240418022000.3524229-1-linmiaohe@huawei.com>
 <20240418022000.3524229-2-linmiaohe@huawei.com>
 <ZiCb_r8O24p8qHIz@localhost.localdomain>
From: Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <a852eb0c-f64e-f1d7-d685-a4e81a6416c6@huawei.com>
Date: Thu, 18 Apr 2024 16:00:42 +0800
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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?

> 
> And if we decide to go with this, I think we are going to need a comment
> in there explaining what is going on like "only init _deferred_list if
> free_huge_folio cannot be call".

Yes, this comment will help.
Thanks.
.

> 
>