linux-kernel - Re: [PATCH 1/2] mm/hugetlb: fix DEBUG_LOCKS_WARN_ON(1) when dissolve_free_hugetlb

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Date: Fri, 19 Apr 2024 10:00:29 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Oscar Salvador <osalvador@...e.de>
CC: <akpm@...ux-foundation.org>, <muchun.song@...ux.dev>, <david@...hat.com>,
	<vbabka@...e.cz>, <willy@...radead.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm/hugetlb: fix DEBUG_LOCKS_WARN_ON(1) when
 dissolve_free_hugetlb_folio()

On 2024/4/18 20:41, Oscar Salvador wrote:
> 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.

That would be really nice. Thanks Oscar.
.

> 
>