linux-kernel - Re: [PATCH] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share

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Message-ID: <3bbfa5ff-cecb-45f8-b1ef-e380cba155a6@oracle.com>
Date: Mon, 15 Sep 2025 09:51:33 -0700
From: jane.chu@...cle.com
To: David Hildenbrand <david@...hat.com>, harry.yoo@...cle.com,
        osalvador@...e.de, liushixin2@...wei.com, muchun.song@...ux.dev,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: fix copy_hugetlb_page_range() to use
 ->pt_share_count


On 9/12/2025 12:31 AM, David Hildenbrand wrote:
> On 11.09.25 21:54, jane.chu@...cle.com wrote:
>>
>> On 9/9/2025 11:45 PM, David Hildenbrand wrote:
>> [..]
>>>> -        /*
>>>> -         * If the pagetables are shared don't copy or take references.
>>>> -         *
>>>> -         * dst_pte == src_pte is the common case of src/dest sharing.
>>>> -         * However, src could have 'unshared' and dst shares with
>>>> -         * another vma. So page_count of ptep page is checked instead
>>>> -         * to reliably determine whether pte is shared.
>>>> -         */
>>>> -        if (page_count(virt_to_page(dst_pte)) > 1) {
>>>> +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
>>>> +        /* If the pagetables are shared don't copy or take
>>>> references. */
>>>
>>> Why remove so much of the original comment?
>>
>> Because, this part of checking has already advanced from the "dst_pte ==
>> src_pte" to "page_count() > 1" to ->pt_share_count > 0, it seems cleaner
>> to just keep an one liner comment.
>> That said, if you feel the comments should be kept, I'd be happy to
>> restore them with a bit revision.
> 
> Well, the comment explains why checking the pte pointers is insufficient 
> and why there is a corner case where the pointers differ but we still 
> want to unshare. :)
> 
> But yeah, I agree that reading the code it's clear: if dst is already 
> shared, just don't do anything.
> 
> I would probably rephrase the comment to something simpler like
> 
> "/* If the pagetables are shared, there is nothing to do. */
> 
> If you resend, please add a comment to the patch description like "While 
> at it, simplify the comment, the details are not actually relevant 
> anymore".
> 

Will do, thanks!

-jane