linux-kernel - Re: [PATCH v3 1/5] hugetlb: use page.private for hugetlb specific page flags

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Message-ID: <c1feff22-115c-ea49-a720-813a86ad9922@oracle.com>
Date:   Wed, 27 Jan 2021 15:01:58 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/5] hugetlb: use page.private for hugetlb specific
 page flags

On 1/27/21 2:20 AM, Michal Hocko wrote:
> [sorry for jumping in late]
> 
> On Fri 22-01-21 11:52:27, Mike Kravetz wrote:
>> As hugetlbfs evolved, state information about hugetlb pages was added.
>> One 'convenient' way of doing this was to use available fields in tail
>> pages.  Over time, it has become difficult to know the meaning or contents
>> of fields simply by looking at a small bit of code.  Sometimes, the
>> naming is just confusing.  For example: The PagePrivate flag indicates
>> a huge page reservation was consumed and needs to be restored if an error
>> is encountered and the page is freed before it is instantiated.  The
>> page.private field contains the pointer to a subpool if the page is
>> associated with one.
> 
> OK, I thought the page.private was abused more than for this very
> specific case.
> 
>> In an effort to make the code more readable, use page.private to contain
>> hugetlb specific page flags.  These flags will have test, set and clear
>> functions similar to those used for 'normal' page flags.  More importantly,
>> an enum of flag values will be created with names that actually reflect
>> their purpose.
> 
> This is definitely a step into the right direction!
> 
>> In this patch,
>> - Create infrastructure for hugetlb specific page flag functions
>> - Move subpool pointer to page[1].private to make way for flags
>>   Create routines with meaningful names to modify subpool field
> 
> This makes some sense as well. It is really important that the primary
> state is stored in the head page. The respective data can be in tail
> pages.
> 
>> - Use new HPageRestoreReserve flag instead of PagePrivate
> 
> Much better! Although wouldn't HPageReserve be sufficient? The flag name
> doesn't really need to tell explicitly what to do with the reserve,
> right? Or would that be too confusing?

Thanks for taking a look.

HPageReserve could be sufficient.  I don't have a strong opinion and was
just trying to add as much meaning to the name as possible.  If you do not
have a strong opinion, I would just leave it as is.

-- 
Mike Kravetz