linux-kernel - Re: [PATCH] mm: Update mask post pxd_clear

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Message-ID: <acdd3003-4a05-4587-93d3-89df3bcd010f@arm.com>
Date: Mon, 17 Mar 2025 12:33:24 +0530
From: Dev Jain <dev.jain@....com>
To: Yeo Reum Yun <YeoReum.Yun@....com>, "jroedel@...e.de" <jroedel@...e.de>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Cc: Ryan Roberts <Ryan.Roberts@....com>, "david@...hat.com"
 <david@...hat.com>, "willy@...radead.org" <willy@...radead.org>,
 "hch@....de" <hch@....de>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] mm: Update mask post pxd_clear_bad()



On 14/03/25 7:57 pm, Yeo Reum Yun wrote:
> Hi Dev,
> 
>>> Since pxd_clear_bad() is an operation changing the state of the page tables,
>>> we should call arch_sync_kernel_mappings() post this.
>>>
>>> Fixes: e80d3909be42 ("mm: track page table modifications in __apply_to_page_range()")
>>> Cc: <stable@...r.kernel.org>
>>> Signed-off-by: Dev Jain <dev.jain@....com>
>>> ---
>>>    mm/memory.c | 4 ++++
>>>    1 file changed, 4 insertions(+)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 78c7ee62795e..9a4a8c710be0 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -2987,6 +2987,7 @@ static int apply_to_pmd_range(struct mm_struct *mm, pud_t *pud,
>>>                        if (!create)
>>>                                continue;
>>>                        pmd_clear_bad(pmd);
>>> +                     *mask = PGTBL_PMD_MODIFIED;
>>
>> Oh well, I guess these should have been *mask |= PGTBL_PMD_MODIFIED.
>>
>>
>>>                }
>>>                err = apply_to_pte_range(mm, pmd, addr, next,
>>>                                         fn, data, create, mask);
>>> @@ -3023,6 +3024,7 @@ static int apply_to_pud_range(struct mm_struct *mm, p4d_t *p4d,
>>>                        if (!create)
>>>                                continue;
>>>                        pud_clear_bad(pud);
>>> +                     *mask = PGTBL_PUD_MODIFIED;
>>>                }
>>>                err = apply_to_pmd_range(mm, pud, addr, next,
>>>                                         fn, data, create, mask);
>>> @@ -3059,6 +3061,7 @@ static int apply_to_p4d_range(struct mm_struct *mm, pgd_t *pgd,
>>>                        if (!create)
>>>                                continue;
>>>                        p4d_clear_bad(p4d);
>>> +                     *mask = PGTBL_P4D_MODIFIED;
>>>                }
>>>                err = apply_to_pud_range(mm, p4d, addr, next,
>>>                                         fn, data, create, mask);
>>> @@ -3095,6 +3098,7 @@ static int __apply_to_page_range(struct mm_struct *mm, unsigned long addr,
>>>                        if (!create)
>>>                                continue;
>>>                        pgd_clear_bad(pgd);
>> +                     mask = PGTBL_PGD_MODIFIED;
>>>                }
>>>                err = apply_to_p4d_range(mm, pgd, addr, next,
>>>                                         fn, data, create, &mask);
> 
> I don't think this wouldn't need.
> the pXd_clear_bad() is only called at creation of each level of page table,
> and when it clear, the following, apply_to_pXd_range() function would be set
> the make properly via pXd_alloc() and apply_to_pte_range().

Makes sense. But pxd_clear_bad() gets called in case of !pxd_none(), so 
while creating, why would the page containing the page table not be 
none? I believe it should be cleared already?

> 
> Thanks.
> 
>