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

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Message-ID: <cde527a6-36cf-8a01-16b9-41e950676e48@suse.cz>
Date:   Tue, 31 Aug 2021 18:37:57 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Miaohe Lin <linmiaohe@...wei.com>,
        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 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.