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

From: Mike Kravetz <mike.kravetz@oracle.com>
To: Michal Hocko <mhocko@suse.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Muchun Song <songmuchun@bytedance.com>,
	David Hildenbrand <david@redhat.com>,
	Oscar Salvador <osalvador@suse.de>,
	Matthew Wilcox <willy@infradead.org>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v3 1/5] hugetlb: use page.private for hugetlb specific page flags
Date: Wed, 27 Jan 2021 15:01:58 -0800	[thread overview]
Message-ID: <c1feff22-115c-ea49-a720-813a86ad9922@oracle.com> (raw)
In-Reply-To: <20210127102035.GF827@dhcp22.suse.cz>

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