linux-kernel - Re: [PATCH] mm: hugetlb: kill set_huge_swap_pte

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Message-ID: <7aee2aeb-acb2-4fe3-90b5-3dd43ee855c2@arm.com>
Date:   Thu, 21 Sep 2023 17:25:44 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Qi Zheng <zhengqi.arch@...edance.com>, mike.kravetz@...cle.com,
        songmuchun@...edance.com, akpm@...ux-foundation.org,
        catalin.marinas@....com, will@...nel.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: hugetlb: kill set_huge_swap_pte_at()

On 26/06/2022 15:57, Qi Zheng wrote:
> The commit e5251fd43007 ("mm/hugetlb: introduce set_huge_swap_pte_at()
> helper") add set_huge_swap_pte_at() to handle swap entries on
> architectures that support hugepages consisting of contiguous ptes.
> And currently the set_huge_swap_pte_at() is only overridden by arm64.
> 
> The set_huge_swap_pte_at() provide a sz parameter to help determine
> the number of entries to be updated. But in fact, all hugetlb swap
> entries contain pfn information, so we can find the corresponding
> folio through the pfn recorded in the swap entry, then the folio_size()
> is the number of entries that need to be updated.
> 
> And considering that users will easily cause bugs by ignoring the
> difference between set_huge_swap_pte_at() and set_huge_pte_at().
> Let's handle swap entries in set_huge_pte_at() and remove the
> set_huge_swap_pte_at(), then we can call set_huge_pte_at()
> anywhere, which simplifies our coding.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>

Hi,

FYI, I discovered a bug in v6.6-rc1 that causes a kernel panic, which I believe
is caused by this change. I've posted a fix along with a detailed explanation at
[1].

[1]
https://lore.kernel.org/linux-arm-kernel/20230921162007.1630149-1-ryan.roberts@arm.com/

Thanks,
Ryan