linux-kernel - Re: [PATCH] mm/vmalloc: fix align value calculation error

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Date:	Fri, 5 Aug 2016 23:48:21 +0800
From:	zijun_hu <zijun_hu@...o.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	tj@...nel.org, hannes@...xchg.org, mhocko@...nel.org,
	minchan@...nel.org, zijun_hu@....com, rientjes@...gle.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/vmalloc: fix align value calculation error

On 2016/8/5 5:24, Andrew Morton wrote:
>>
>> it causes double align requirement for __get_vm_area_node() if parameter
>> size is power of 2 and VM_IOREMAP is set in parameter flags
>>
>> it is fixed by handling the specail case manually due to lack of
>> get_count_order() for long parameter
>>
>> ...
>>
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1357,11 +1357,16 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
>>  {
>>  	struct vmap_area *va;
>>  	struct vm_struct *area;
>> +	int ioremap_size_order;
>>  
>>  	BUG_ON(in_interrupt());
>> -	if (flags & VM_IOREMAP)
>> -		align = 1ul << clamp_t(int, fls_long(size),
>> -				       PAGE_SHIFT, IOREMAP_MAX_ORDER);
>> +	if (flags & VM_IOREMAP) {
>> +		ioremap_size_order = fls_long(size);
>> +		if (is_power_of_2(size) && size != 1)
>> +			ioremap_size_order--;
>> +		align = 1ul << clamp_t(int, ioremap_size_order, PAGE_SHIFT,
>> +				IOREMAP_MAX_ORDER);
>> +	}
>>  
>>  	size = PAGE_ALIGN(size);
>>  	if (unlikely(!size))
> 
> I'm having trouble with this, and a more complete description would
> have helped!
> 
> As far as I can tell, the current code will decide the following:
> 
> size=0x10000: alignment=0x10000
> size=0x0f000: alignment=0x8000
> 

no, the current code doesn't achieve the above results as shown below
size=0x10000 -> fls_long(0x10000)=17 -> alignment=0x20000
size=0x0f000 -> fls_long(0x0f000)=16 -> alignment=0x10000
it is wrong for power of 2 value such as size=0x10000

> And your patch will change it so that
> 
> size=0x10000: alignment=0x8000
> size=0x0f000: alignment=0x8000
> 
> Correct?
>

no, my patch will results in the following calculations
size=0x10000: alignment=0x10000
size=0x0f000: alignment=0x10000

> If so, I'm struggling to see the sense in this.  Shouldn't we be
> changing things so that
> 
> size=0x10000: alignment=0x10000
> size=0x0f000: alignment=0x10000
> 
> ?
okay, it is the aim of my patch as explained above
i provide another solution as shown below
i appreciate it since it is more canonical
please help to review and apply it kindly

>From 1fa79b706735908b2c7aed635dcad7ed9c0a2a87 Mon Sep 17 00:00:00 2001
From: zijun_hu <zijun_hu@....com>
Date: Fri, 5 Aug 2016 22:10:07 +0800
Subject: [PATCH 1/1] mm/vmalloc: fix align value calculation error

it causes double align requirement for __get_vm_area_node() if parameter
size is power of 2 and VM_IOREMAP is set in parameter flags

get_order_long() is implemented and used instead of fls_long() for
fixing the bug

Signed-off-by: zijun_hu <zijun_hu@....com>
---
 include/linux/bitops.h | 17 +++++++++++++++++
 mm/vmalloc.c           |  2 +-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 299e76b..c18448d 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -192,6 +192,23 @@ static inline unsigned fls_long(unsigned long l)
 }
 
 /**
+ * get_order_long - get order after rounding @l up to power of 2
+ * @l: parameter
+ *
+ * it is same as get_count_order() but long type parameter
+ * or 0 is returned if @l == 0UL
+ */
+static inline int get_order_long(unsigned long l)
+{
+	if (l == 0UL)
+		return 0;
+	else if (l & (l - 1UL))
+		return fls_long(l);
+	else
+		return fls_long(l) - 1;
+}
+
+/**
  * __ffs64 - find first set bit in a 64 bit word
  * @word: The 64 bit word
  *
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 91f44e7..7d717f3 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1360,7 +1360,7 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
 
 	BUG_ON(in_interrupt());
 	if (flags & VM_IOREMAP)
-		align = 1ul << clamp_t(int, fls_long(size),
+		align = 1ul << clamp_t(int, get_order_long(size),
 				       PAGE_SHIFT, IOREMAP_MAX_ORDER);
 
 	size = PAGE_ALIGN(size);
-- 
1.9.1