linux-kernel - Re: [PATCH 06/14] mm: handle_pte_fault() use pte_offset_map_maywrite

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Message-ID: <83142dc8-edcf-4e47-8215-8b359a2b7156@bytedance.com>
Date: Wed, 21 Aug 2024 18:03:30 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: LEROY Christophe <christophe.leroy2@...soprasteria.com>,
 "hughd@...gle.com" <hughd@...gle.com>,
 "willy@...radead.org" <willy@...radead.org>,
 "muchun.song@...ux.dev" <muchun.song@...ux.dev>,
 "vbabka@...nel.org" <vbabka@...nel.org>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
 "rppt@...nel.org" <rppt@...nel.org>,
 "vishal.moola@...il.com" <vishal.moola@...il.com>,
 "peterx@...hat.com" <peterx@...hat.com>,
 "ryan.roberts@....com" <ryan.roberts@....com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 06/14] mm: handle_pte_fault() use
 pte_offset_map_maywrite_nolock()



On 2024/8/21 17:53, David Hildenbrand wrote:
> On 21.08.24 11:51, Qi Zheng wrote:
>>
>>
>> On 2024/8/21 17:41, David Hildenbrand wrote:
>>> On 21.08.24 11:24, Qi Zheng wrote:
>>>>
>>>>
>>>> On 2024/8/21 17:17, LEROY Christophe wrote:
>>>>>
>>>>>
>>>>> Le 21/08/2024 à 10:18, Qi Zheng a écrit :
>>>>>> In handle_pte_fault(), we may modify the vmf->pte after acquiring the
>>>>>> vmf->ptl, so convert it to using pte_offset_map_maywrite_nolock(). 
>>>>>> But
>>>>>> since we already do the pte_same() check, so there is no need to get
>>>>>> pmdval to do pmd_same() check, just pass NULL to pmdvalp parameter.
>>>>>>
>>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>>>>> ---
>>>>>>      mm/memory.c | 9 +++++++--
>>>>>>      1 file changed, 7 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>>> index 93c0c25433d02..d3378e98faf13 100644
>>>>>> --- a/mm/memory.c
>>>>>> +++ b/mm/memory.c
>>>>>> @@ -5504,9 +5504,14 @@ static vm_fault_t handle_pte_fault(struct
>>>>>> vm_fault *vmf)
>>>>>>               * pmd by anon khugepaged, since that takes mmap_lock in
>>>>>> write
>>>>>>               * mode; but shmem or file collapse to THP could still
>>>>>> morph
>>>>>>               * it into a huge pmd: just retry later if so.
>>>>>> +         *
>>>>>> +         * Use the maywrite version to indicate that vmf->pte 
>>>>>> will be
>>>>>> +         * modified, but since we will use pte_same() to detect the
>>>>>> +         * change of the pte entry, there is no need to get pmdval.
>>>>>>               */
>>>>>> -        vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>>>> -                         vmf->address, &vmf->ptl);
>>>>>> +        vmf->pte = pte_offset_map_maywrite_nolock(vmf->vma->vm_mm,
>>>>>> +                              vmf->pmd, vmf->address,
>>>>>> +                              NULL, &vmf->ptl);
>>>
>>> I think we discussed that passing NULL should be forbidden for that
>>> function.
>>
>> Yes, but for some maywrite case, there is no need to get pmdval to
>> do pmd_same() check. So I passed NULL and added a comment to
>> explain this.
> 
> I wonder if it's better to pass a dummy variable instead. One has to 
> think harder why that is required compared to blindly passing "NULL" :)

You are afraid that subsequent caller will abuse this function, right?
My initial concern was that this would add a useless local vaiable, but
perhaps that is not a big deal.

Both are fine for me. ;)

>