linux-kernel - Re: [PATCH v3 1/4] mm/hwpoison: delete all entries before traversal in __folio_free_raw

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Date:   Sat, 8 Jul 2023 10:40:21 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Jiaqi Yan <jiaqiyan@...gle.com>, <mike.kravetz@...cle.com>,
        <naoya.horiguchi@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <songmuchun@...edance.com>, <shy828301@...il.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <duenwen@...gle.com>, <axelrasmussen@...gle.com>,
        <jthoughton@...gle.com>
Subject: Re: [PATCH v3 1/4] mm/hwpoison: delete all entries before traversal
 in __folio_free_raw_hwp

On 2023/7/8 4:19, Jiaqi Yan wrote:
> Traversal on llist (e.g. llist_for_each_safe) is only safe AFTER entries
> are deleted from the llist. Correct the way __folio_free_raw_hwp deletes
> and frees raw_hwp_page entries in raw_hwp_list: first llist_del_all, then
> kfree within llist_for_each_safe.
> 
> As of today, concurrent adding, deleting, and traversal on raw_hwp_list
> from hugetlb.c and/or memory-failure.c are fine with each other. Note

I think there's a race on freeing the raw_hwp_list between unpoison_memory and __update_and_free_hugetlb_folio:

  unpoison_memory		__update_and_free_hugetlb_folio
    				if (folio_test_hwpoison)
    	  			    folio_clear_hugetlb_hwpoison
    folio_free_raw_hwp		      folio_free_raw_hwp
    folio_test_clear_hwpoison

unpoison_memory and __update_and_free_hugetlb_folio can traverse and free the raw_hwp_list
at the same time. And I believe your patch will fix the problem. Thanks.

> this is guaranteed partly by the lock-free nature of llist, and partly
> by holding hugetlb_lock and/or mf_mutex. For example, as llist_del_all
> is lock-free with itself, folio_clear_hugetlb_hwpoison()s from
> __update_and_free_hugetlb_folio and memory_failure won't need explicit
> locking when freeing the raw_hwp_list. New code that manipulates
> raw_hwp_list must be careful to ensure the concurrency correctness.
> 
> Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
> Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>
> Signed-off-by: Jiaqi Yan <jiaqiyan@...gle.com>

Anyway, this patch looks good to me.

Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
Thanks.