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

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Date:   Tue, 31 Aug 2021 09:56:02 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <vbabka@...e.cz>,
        <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 2021/8/30 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?

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().
> .
>