The patch titled
Subject: mm: pgtable: make ptep_clear() non-atomic
has been added to the -mm mm-unstable branch. Its filename is
mm-pgtable-make-ptep_clear-non-atomic.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-pgtable-make-ptep_clear-non-atomic.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Subject: mm: pgtable: make ptep_clear() non-atomic
Date: Fri, 22 Nov 2024 15:36:52 +0800
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.
Currently ptep_clear() is only used in debug code and in khugepaged
collapse paths, which are fairly expensive. So the cost of an extra atomic
RMW operation does not matter. But this may be used for other paths in the
future. After all, for the present pte entry, we need to call ptep_clear()
instead of pte_clear() to ensure that PAGE_TABLE_CHECK works properly.
So to be more precise, just calling ptep_get() and pte_clear() in the
ptep_clear().
Link: https://lkml.kernel.org/r/20241122073652.54030-1-zhengqi.arch@xxxxxxxxxxxxx
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Tong Tiangen <tongtiangen@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/pgtable.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
--- a/include/linux/pgtable.h~mm-pgtable-make-ptep_clear-non-atomic
+++ a/include/linux/pgtable.h
@@ -533,7 +533,14 @@ static inline void clear_young_dirty_pte
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);
+ /*
+ * No need for ptep_get_and_clear(): page table check doesn't care about
+ * any bits that could have been set by HW concurrently.
+ */
+ page_table_check_pte_clear(mm, pte);
}
#ifdef CONFIG_GUP_GET_PXX_LOW_HIGH
_
Patches currently in -mm which might be from zhengqi.arch@xxxxxxxxxxxxx are
mm-pgtable-make-ptep_clear-non-atomic.patch
