linux-kernel - Re: [PATCH 6/6] mm/page_alloc.c: avoid allocating highmem pages via alloc_pages_exact

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Date:   Wed, 1 Sep 2021 16:05:44 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <sfr@...b.auug.org.au>,
        <peterz@...radead.org>, <mgorman@...hsingularity.net>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] mm/page_alloc.c: avoid allocating highmem pages via
 alloc_pages_exact_nid()

On 2021/9/1 0:37, Vlastimil Babka wrote:
> On 8/31/21 03:56, Miaohe Lin wrote:
>> On 2021/8/30 22:24, Matthew Wilcox wrote:
>>> On Mon, Aug 30, 2021 at 10:10:51PM +0800, Miaohe Lin wrote:
>>>> Don't use with __GFP_HIGHMEM because page_address() cannot represent
>>>> highmem pages without kmap(). Newly allocated pages would leak as
>>>> page_address() will return NULL for highmem pages here. But It works
>>>> now because the only caller does not specify __GFP_HIGHMEM now.
>>>
>>> This is a misunderstanding of how alloc_pages_exact() /
>>> alloc_pages_exact_nid() work.  You simply can't call them with
>>> GFP_HIGHMEM.
>>>
>>
>> Yep, they can't work with GFP_HIGHMEM. So IMO it might be better to
>> get rid of GFP_HIGHMEM explicitly or add a comment to clarify this
>> situation to avoid future misbehavior. But this may be a unnecessary
>> worry... Do you prefer to not change anything here?
> 
> I agree with the suggestion below...
> 
>> Many thanks.
>>
>>> If you really must change anything here,
>>> s/__GFP_COMP/(__GFP_COMP|__GFP_HIGHMEM)/g throughout both
>>> alloc_pages_exact() and alloc_pages_exact_nid().
> 
> ... which means __GFP_HIGHMEM would be stripped and additionally there would
> be a warning.
> 

Looks good for me. Will do. Many thanks!

> .
>