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

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Message-ID: <B65776A4-D434-4D9F-9C42-1C45DAE5A72A@nvidia.com>
Date: Wed, 04 Dec 2024 11:58:38 -0500
From: Zi Yan <ziy@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
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 4 Dec 2024, at 11:29, Matthew Wilcox wrote:

> 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"?

I mean if kmap address goes into a different cache line than userspace
address, a cache flush is needed to make sure data is visible to
userspace.

>
>> 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.

So this fix will not work, since it is possible that first kmap and second
kmap have different pages_do_alias() return values.

Another way would be to make a special case for mips, like below.
But that looks ugly, let me think about it more.

diff --git a/arch/mips/include/asm/page.h b/arch/mips/include/asm/page.h
index bc3e3484c1bf..ef3c6f0b9159 100644
--- a/arch/mips/include/asm/page.h
+++ b/arch/mips/include/asm/page.h
@@ -95,6 +95,19 @@ struct vm_area_struct;
 extern void copy_user_highpage(struct page *to, struct page *from,
 	unsigned long vaddr, struct vm_area_struct *vma);

+struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
+ 				   unsigned long vaddr)
+ {
+	struct folio *folio;
+
+	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr);
+	if (folio)
+		clear_user_highpage(&folio->page, vaddr);
+
+	return folio;
+ }
+#define vma_alloc_zeroed_movable_folio vma_alloc_zeroed_movable_folio
+
 #define __HAVE_ARCH_COPY_USER_HIGHPAGE

 /*
diff --git a/mm/internal.h b/mm/internal.h
index cb8d8e8e3ffa..d513fa683aa3 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1287,7 +1287,8 @@ void touch_pmd(struct vm_area_struct *vma, unsigned long addr,

 static inline bool alloc_zeroed(void)
 {
-	return static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
+	return !IS_ENABLED(CONFIG_MIPS) &&
+		static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
 			&init_on_alloc);
 }


Best Regards,
Yan, Zi