* [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush()
@ 2021-02-03 8:41 Miaohe Lin
2021-02-04 12:36 ` Matthew Wilcox
0 siblings, 1 reply; 3+ messages in thread
From: Miaohe Lin @ 2021-02-03 8:41 UTC (permalink / raw)
To: akpm; +Cc: linux-mm, linux-kernel, linmiaohe
The developer will have trouble figuring out why the BUG actually triggered
when there is a complex expression in the VM_BUG_ON. Because we can only
identify the condition triggered BUG via line number provided by VM_BUG_ON.
Optimize this by spliting such a complex expression into two simple
conditions.
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
mm/pgtable-generic.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index fa1375f3e3b2..c2210e1cdb51 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -135,8 +135,9 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
{
pmd_t pmd;
VM_BUG_ON(address & ~HPAGE_PMD_MASK);
- VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
- !pmd_devmap(*pmdp)));
+ VM_BUG_ON(!pmd_present(*pmdp));
+ /* Below assumes pmd_present() is true */
+ VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
return pmd;
--
2.19.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush()
2021-02-03 8:41 [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush() Miaohe Lin
@ 2021-02-04 12:36 ` Matthew Wilcox
2021-02-05 3:22 ` Miaohe Lin
0 siblings, 1 reply; 3+ messages in thread
From: Matthew Wilcox @ 2021-02-04 12:36 UTC (permalink / raw)
To: Miaohe Lin; +Cc: akpm, linux-mm, linux-kernel
On Wed, Feb 03, 2021 at 03:41:37AM -0500, Miaohe Lin wrote:
> The developer will have trouble figuring out why the BUG actually triggered
> when there is a complex expression in the VM_BUG_ON. Because we can only
> identify the condition triggered BUG via line number provided by VM_BUG_ON.
> Optimize this by spliting such a complex expression into two simple
> conditions.
> pmd_t pmd;
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> - VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
> - !pmd_devmap(*pmdp)));
> + VM_BUG_ON(!pmd_present(*pmdp));
> + /* Below assumes pmd_present() is true */
> + VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
This is not a complex condition. We're in the huge PMD handling case
and we're looking at a PMD which either isn't present or isn't huge.
It might be useful to print out the PMD in such a case, but splitting
this into the two cases of pmd-not-present and pmd-isn't-huge isn't
particularly useful.
I think you know that, or you wouldn't feel the need to put in a
comment explaining it!
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush()
2021-02-04 12:36 ` Matthew Wilcox
@ 2021-02-05 3:22 ` Miaohe Lin
0 siblings, 0 replies; 3+ messages in thread
From: Miaohe Lin @ 2021-02-05 3:22 UTC (permalink / raw)
To: Matthew Wilcox; +Cc: akpm, linux-mm, linux-kernel
Hi:
On 2021/2/4 20:36, Matthew Wilcox wrote:
> On Wed, Feb 03, 2021 at 03:41:37AM -0500, Miaohe Lin wrote:
>> The developer will have trouble figuring out why the BUG actually triggered
>> when there is a complex expression in the VM_BUG_ON. Because we can only
>> identify the condition triggered BUG via line number provided by VM_BUG_ON.
>> Optimize this by spliting such a complex expression into two simple
>> conditions.
>
>> pmd_t pmd;
>> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> - VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
>> - !pmd_devmap(*pmdp)));
>> + VM_BUG_ON(!pmd_present(*pmdp));
>> + /* Below assumes pmd_present() is true */
>> + VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
>
> This is not a complex condition. We're in the huge PMD handling case
> and we're looking at a PMD which either isn't present or isn't huge.
> It might be useful to print out the PMD in such a case, but splitting
> this into the two cases of pmd-not-present and pmd-isn't-huge isn't
> particularly useful.
>
Many thanks for your time. You are right that it would be more helpful if we
had a VM_BUG_ON_PMD() that we could print the pmd's value and permit diagnosis
from that. I think splitting this into the two cases is the best we can do now
while lacking of such helper.
> I think you know that, or you wouldn't feel the need to put in a
> comment explaining it!
> .
>
Thanks again.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2021-02-05 3:23 UTC | newest]
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2021-02-03 8:41 [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush() Miaohe Lin
2021-02-04 12:36 ` Matthew Wilcox
2021-02-05 3:22 ` Miaohe Lin
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