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

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Message-ID: <ef40c2ef-e4a4-4b02-85b8-a930285a3d0e@bytedance.com>
Date: Wed, 21 Aug 2024 17:51:21 +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 17:41, 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.

> 
>>>
>>> This might be the demonstration that the function name is becoming 
>>> too long.
>>>
>>> Can you find shorter names ?
>>
>> Maybe use abbreviations?
>>
>> pte_offset_map_ro_nolock()
>> pte_offset_map_rw_nolock()
> 
> At least the "ro" is better, but "rw" does not express the "maywrite" -- 
> because without taking the lock we are not allowed to write. But maybe 
> "rw" is good enough for that if we document it properly.

OK, will change to it in the next version.

> 
> And you can use up to 100 characters, if it helps readability

Got it.

>