linux-kernel - Re: [PATCH] mm: avoid zeroing user movable page twice with init_on

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Message-ID: <Z1CDbrrTn6RgNmYn@casper.infradead.org>
Date: Wed, 4 Dec 2024 16:29:34 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Zi Yan <ziy@...dia.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
	Geert Uytterhoeven <geert@...ux-m68k.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	John Hubbard <jhubbard@...dia.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Alexander Potapenko <glider@...gle.com>,
	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
	linux-mips@...r.kernel.org
Subject: Re: [PATCH] mm: avoid zeroing user movable page twice with
 init_on_alloc=1

On Wed, Dec 04, 2024 at 11:16:51AM -0500, Zi Yan wrote:
> > So maybe the clearing done as part of page allocator isn't enough here.
> >
> Basically, mips needs to flush data cache if kmap address is aliased to

People use "aliased" in contronym ways.  Do you mean "has a
non-congruent alias" or "has a congruent alias"?

> userspace address. This means when mips has THP on, the patch below
> is not enough to fix the issue.
> 
> In post_alloc_hook(), it does not make sense to pass userspace address
> in to determine whether to flush dcache or not.
> 
> One way to fix it is to add something like arch_userpage_post_alloc()
> to flush dcache if kmap address is aliased to userspace address.
> But my questions are that
> 1) if kmap address will always be the same for two separate kmap_local() calls,

No.  It just takes the next address in the stack.

> 2) how much overheads the additional kmap_local() and kunmap_local() have.

That's going to be a per-arch question ...