On 18.09.24 08:32, Anshuman Khandual wrote:
On 9/17/24 15:58, David Hildenbrand wrote:
On 17.09.24 09:31, Anshuman Khandual wrote:
Convert PTE accesses via ptep_get() helper that defaults as READ_ONCE() but
also provides the platform an opportunity to override when required. This
stores read page table entry value in a local variable which can be used in
multiple instances there after. This helps in avoiding multiple memory load
operations as well possible race conditions.
Please make it clearer in the subject+description that this really only involves set_pte_safe().
I will update the commit message with some thing like this.
mm: Use ptep_get() in set_pte_safe()
This converts PTE accesses in set_pte_safe() via ptep_get() helper which
defaults as READ_ONCE() but also provides the platform an opportunity to
override when required. This stores read page table entry value in a local
variable which can be used in multiple instances there after. This helps
in avoiding multiple memory load operations as well as some possible race
conditions.
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
---
include/linux/pgtable.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 2a6a3cccfc36..547eeae8c43f 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1060,7 +1060,8 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b)
*/
#define set_pte_safe(ptep, pte) \
({ \
- WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \
+ pte_t __old = ptep_get(ptep); \
+ WARN_ON_ONCE(pte_present(__old) && !pte_same(__old, pte)); \
set_pte(ptep, pte); \
})
I don't think this is necessary. PTE present cannot flip concurrently, that's the whole reason of the "safe" part after all.
Which is not necessary ? Converting de-references to ptep_get() OR caching
the page table read value in a local variable ? ptep_get() conversion also
serves the purpose providing an opportunity for platform to override.
Which arch override are you thinking of where this change here would
make a real difference? Would it even make a difference with cont-pte on
arm?
Can we just move these weird set_pte/pmd_safe() stuff to x86 init code and be done with it? Then it's also clear *where* it is getting used and for which reason.
set_pte/pmd_safe() can be moved to x86 platform - as that is currently the
sole user for these helpers. But because set_pgd_safe() gets used in riscv
platform, just wondering would it be worth moving only the pte/pmd helpers
but not the pgd one ?
My take would be just to move them where they are used, and possibly
even inlining them.
The point is that it's absolutely underdocumented what "_safe" is
supposed to be here, and I don't really see the reason to have this in
common code (making the common API more complicated).
--
Cheers,
David / dhildenb
