linux-kernel - Re: [PATCH v3 2/3] mm/gup: remove unneeded checking in follow_page

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Message-ID: <f31be5fd-601f-4742-aa76-e7314b907cdb@redhat.com>
Date: Tue, 8 Apr 2025 11:15:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>, Baoquan He <bhe@...hat.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, mingo@...nel.org,
 yanjun.zhu@...ux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] mm/gup: remove unneeded checking in
 follow_page_pte()

On 08.04.25 11:04, Oscar Salvador wrote:
> On Mon, Apr 07, 2025 at 11:03:05AM +0800, Baoquan He wrote:
>> In __get_user_pages(), it will traverse page table and take a reference
>> to the page the given user address corresponds to if GUP_GET or GUP_PIN
>> is set. However, it's not supported both GUP_GET and GUP_PIN are set.
>> Even though this check need be done, it should be done earlier, but not
>> doing it till entering into follow_page_pte() and failed.
>>
>> Furthermore, this checking has been done in is_valid_gup_args() and all
>> external users of __get_user_pages() will call is_valid_gup_args() to
>> catch the illegal setting. We don't need to worry about internal users
>> of __get_user_pages() because the gup_flags are set by MM code correctly.
>>
>> Here remove the checking in follow_page_pte(), and add VM_WARN_ON_ONCE()
>> to catch the possible exceptional setting just in case.
>>
>> And also change the VM_BUG_ON to VM_WARN_ON_ONCE() for checking
>> (!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN))) because the checking
>> has been done in is_valid_gup_args() for external users of __get_user_pages().
> 
> I will also note that you changed the flags check to VM_WARN_ON_ONCE.
> I guess this is fine as we have the WARN_ON_ONCE version in
> is_valid_gup_args().
> 

Yes, that was my reasoning when I proposed it.

>> Signed-off-by: Baoquan He <bhe@...hat.com>
> 
> LGTM,
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> 
> 

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb