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Message-ID: <c0f64fa7-81fd-4691-86b5-2ad39ba9d8a7@bytedance.com>
Date: Wed, 6 Nov 2024 11:09:47 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Jonathan Corbet <corbet@....net>, Andrew Morton <akpm@...ux-foundation.org>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Alice Ryhl <aliceryhl@...gle.com>,
Boqun Feng <boqun.feng@...il.com>, Matthew Wilcox <willy@...radead.org>,
Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [RFC PATCH] docs/mm: add VMA locks documentation
Hi Jann,
On 2024/11/5 05:29, Jann Horn wrote:
> On Mon, Nov 4, 2024 at 5:42 PM Lorenzo Stoakes
[...]
>
> I think it's important to know about the existence of hardware writes
> because it means you need atomic operations when making changes to
> page tables. Like, for example, in many cases when changing a present
> PTE, you can't even use READ_ONCE()/WRITE_ONCE() for PTEs and need
> atomic RMW operations instead - see for example ptep_get_and_clear(),
> which is basically implemented in arch code as an atomic xchg so that
> it can't miss concurrent A/D bit updates.
>
Totally agree! But I noticed before that ptep_clear() doesn't seem
to need atomic operations because it doesn't need to care about the
A/D bit.
I once looked at the history of how the ptep_clear() was introduced.
If you are interested, you can take a look at my local draft below.
Maybe I missed something.
```
mm: pgtable: make ptep_clear() non-atomic
In the generic ptep_get_and_clear() implementation, it is just a simple
combination of ptep_get() and pte_clear(). But for some architectures
(such as x86 and arm64, etc), the hardware will modify the A/D bits
of the
page table entry, so the ptep_get_and_clear() needs to be overwritten
and implemented as an atomic operation to avoid contention, which has a
performance cost.
The commit d283d422c6c4 ("x86: mm: add x86_64 support for page table
check") adds the ptep_clear() on the x86, and makes it call
ptep_get_and_clear() when CONFIG_PAGE_TABLE_CHECK is enabled. The page
table check feature does not actually care about the A/D bits, so only
ptep_get() + pte_clear() should be called. But considering that the
page
table check is a debug option, this should not have much of an impact.
But then the commit de8c8e52836d ("mm: page_table_check: add hooks to
public helpers") changed ptep_clear() to unconditionally call
ptep_get_and_clear(), so that the CONFIG_PAGE_TABLE_CHECK check can be
put into the page table check stubs (in
include/linux/page_table_check.h).
This also cause performance loss to the kernel without
CONFIG_PAGE_TABLE_CHECK enabled, which doesn't make sense.
To fix it, just calling ptep_get() and pte_clear() in the ptep_clear().
Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 117b807e3f894..2ace92293f5f5 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -506,7 +506,10 @@ static inline void clear_young_dirty_ptes(struct
vm_area_struct *vma,
static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
pte_t *ptep)
{
- ptep_get_and_clear(mm, addr, ptep);
+ pte_t pte = ptep_get(ptep);
+
+ pte_clear(mm, addr, ptep);
+ page_table_check_pte_clear(mm, pte);
}
```
Thanks!
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