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

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Message-ID: <YSzqDt/13YbOAyo4@casper.infradead.org>
Date:   Mon, 30 Aug 2021 15:24:14 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
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 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.

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().