[PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
calculating here will generate an unexpected result. In addition, if
PAGE_SHIFT > 20, The memory size represented by numentries was already
integral multiple of 1MB.

Signed-off-by: Jerry <uulinux@gmail.com>
---
 mm/page_alloc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..cd41797 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
 	if (!numentries) {
 		/* round applicable memory size up to nearest megabyte */
 		numentries = nr_kernel_pages;
-		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
-		numentries >>= 20 - PAGE_SHIFT;
-		numentries <<= 20 - PAGE_SHIFT;
+		if (20 > PAGE_SHIFT) {
+			numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
+			numentries >>= 20 - PAGE_SHIFT;
+			numentries <<= 20 - PAGE_SHIFT;
+		}
 
 		/* limit to 1 bucket per 2^scale bytes of low memory */
 		if (scale > PAGE_SHIFT)
-- 
1.8.1.5

[PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
calculating here will generate an unexpected result. In addition, if
PAGE_SHIFT > 20, The memory size represented by numentries was already
integral multiple of 1MB.

Signed-off-by: Jerry <uulinux@gmail.com>
---
 mm/page_alloc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..cd41797 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
 	if (!numentries) {
 		/* round applicable memory size up to nearest megabyte */
 		numentries = nr_kernel_pages;
-		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
-		numentries >>= 20 - PAGE_SHIFT;
-		numentries <<= 20 - PAGE_SHIFT;
+		if (20 > PAGE_SHIFT) {
+			numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
+			numentries >>= 20 - PAGE_SHIFT;
+			numentries <<= 20 - PAGE_SHIFT;
+		}
 
 		/* limit to 1 bucket per 2^scale bytes of low memory */
 		if (scale > PAGE_SHIFT)
-- 
1.8.1.5

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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[John Stoffel]

Jerry> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
Jerry> calculating here will generate an unexpected result. In addition, if
Jerry> PAGE_SHIFT > 20, The memory size represented by numentries was already
Jerry> integral multiple of 1MB.

Why this magic number of 20?  Please explain it better and replace it
was a #define that means something here.  


Jerry> Signed-off-by: Jerry <uulinux@gmail.com>
Jerry> ---
Jerry>  mm/page_alloc.c | 8 +++++---
Jerry>  1 file changed, 5 insertions(+), 3 deletions(-)

Jerry> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
Jerry> index b100255..cd41797 100644
Jerry> --- a/mm/page_alloc.c
Jerry> +++ b/mm/page_alloc.c
Jerry> @@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
Jerry>  	if (!numentries) {
Jerry>  		/* round applicable memory size up to nearest megabyte */
Jerry>  		numentries = nr_kernel_pages;
Jerry> -		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
Jerry> -		numentries >>= 20 - PAGE_SHIFT;
Jerry> -		numentries <<= 20 - PAGE_SHIFT;
Jerry> +		if (20 > PAGE_SHIFT) {
Jerry> +			numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
Jerry> +			numentries >>= 20 - PAGE_SHIFT;
Jerry> +			numentries <<= 20 - PAGE_SHIFT;
Jerry> +		}
 
Jerry>  		/* limit to 1 bucket per 2^scale bytes of low memory */
Jerry>  		if (scale > PAGE_SHIFT)
Jerry> -- 
Jerry> 1.8.1.5

Jerry> --
Jerry> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
Jerry> the body of a message to majordomo@vger.kernel.org
Jerry> More majordomo info at  http://vger.kernel.org/majordomo-info.html
Jerry> Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[John Stoffel]

Jerry> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
Jerry> calculating here will generate an unexpected result. In addition, if
Jerry> PAGE_SHIFT > 20, The memory size represented by numentries was already
Jerry> integral multiple of 1MB.

Why this magic number of 20?  Please explain it better and replace it
was a #define that means something here.  


Jerry> Signed-off-by: Jerry <uulinux@gmail.com>
Jerry> ---
Jerry>  mm/page_alloc.c | 8 +++++---
Jerry>  1 file changed, 5 insertions(+), 3 deletions(-)

Jerry> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
Jerry> index b100255..cd41797 100644
Jerry> --- a/mm/page_alloc.c
Jerry> +++ b/mm/page_alloc.c
Jerry> @@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
Jerry>  	if (!numentries) {
Jerry>  		/* round applicable memory size up to nearest megabyte */
Jerry>  		numentries = nr_kernel_pages;
Jerry> -		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
Jerry> -		numentries >>= 20 - PAGE_SHIFT;
Jerry> -		numentries <<= 20 - PAGE_SHIFT;
Jerry> +		if (20 > PAGE_SHIFT) {
Jerry> +			numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
Jerry> +			numentries >>= 20 - PAGE_SHIFT;
Jerry> +			numentries <<= 20 - PAGE_SHIFT;
Jerry> +		}
 
Jerry>  		/* limit to 1 bucket per 2^scale bytes of low memory */
Jerry>  		if (scale > PAGE_SHIFT)
Jerry> -- 
Jerry> 1.8.1.5

Jerry> --
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Jerry> Please read the FAQ at  http://www.tux.org/lkml/

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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

> Jerry> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> Jerry> calculating here will generate an unexpected result. In addition, if
> Jerry> PAGE_SHIFT > 20, The memory size represented by numentries was already
> Jerry> integral multiple of 1MB.
>
> Why this magic number of 20?  Please explain it better and replace it
> was a #define that means something here.

Because 2^20 = 1MB.

The intention of previous code is "/* round applicable memory size up
to nearest megabyte */".

>
> Jerry> Signed-off-by: Jerry <uulinux@gmail.com>
> Jerry> ---
> Jerry>  mm/page_alloc.c | 8 +++++---
> Jerry>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> Jerry> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> Jerry> index b100255..cd41797 100644
> Jerry> --- a/mm/page_alloc.c
> Jerry> +++ b/mm/page_alloc.c
> Jerry> @@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
> Jerry>          if (!numentries) {
> Jerry>                  /* round applicable memory size up to nearest megabyte */
> Jerry>                  numentries = nr_kernel_pages;
> Jerry> -                numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
> Jerry> -                numentries >>= 20 - PAGE_SHIFT;
> Jerry> -                numentries <<= 20 - PAGE_SHIFT;
> Jerry> +                if (20 > PAGE_SHIFT) {
> Jerry> +                        numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
> Jerry> +                        numentries >>= 20 - PAGE_SHIFT;
> Jerry> +                        numentries <<= 20 - PAGE_SHIFT;
> Jerry> +                }
>
> Jerry>                  /* limit to 1 bucket per 2^scale bytes of low memory */
> Jerry>                  if (scale > PAGE_SHIFT)
> Jerry> --
> Jerry> 1.8.1.5
>
> Jerry> --
> Jerry> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> Jerry> the body of a message to majordomo@vger.kernel.org
> Jerry> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Jerry> Please read the FAQ at  http://www.tux.org/lkml/



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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

> Jerry> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> Jerry> calculating here will generate an unexpected result. In addition, if
> Jerry> PAGE_SHIFT > 20, The memory size represented by numentries was already
> Jerry> integral multiple of 1MB.
>
> Why this magic number of 20?  Please explain it better and replace it
> was a #define that means something here.

Because 2^20 = 1MB.

The intention of previous code is "/* round applicable memory size up
to nearest megabyte */".

>
> Jerry> Signed-off-by: Jerry <uulinux@gmail.com>
> Jerry> ---
> Jerry>  mm/page_alloc.c | 8 +++++---
> Jerry>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> Jerry> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> Jerry> index b100255..cd41797 100644
> Jerry> --- a/mm/page_alloc.c
> Jerry> +++ b/mm/page_alloc.c
> Jerry> @@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
> Jerry>          if (!numentries) {
> Jerry>                  /* round applicable memory size up to nearest megabyte */
> Jerry>                  numentries = nr_kernel_pages;
> Jerry> -                numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
> Jerry> -                numentries >>= 20 - PAGE_SHIFT;
> Jerry> -                numentries <<= 20 - PAGE_SHIFT;
> Jerry> +                if (20 > PAGE_SHIFT) {
> Jerry> +                        numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
> Jerry> +                        numentries >>= 20 - PAGE_SHIFT;
> Jerry> +                        numentries <<= 20 - PAGE_SHIFT;
> Jerry> +                }
>
> Jerry>                  /* limit to 1 bucket per 2^scale bytes of low memory */
> Jerry>                  if (scale > PAGE_SHIFT)
> Jerry> --
> Jerry> 1.8.1.5
>
> Jerry> --
> Jerry> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> Jerry> the body of a message to majordomo@vger.kernel.org
> Jerry> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Jerry> Please read the FAQ at  http://www.tux.org/lkml/



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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Andrew Morton]

On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:

> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> calculating here will generate an unexpected result. In addition, if
> PAGE_SHIFT > 20, The memory size represented by numentries was already
> integral multiple of 1MB.
> 

If you tell me that you have a machine which has PAGE_SIZE=2MB and this
was the only problem which prevented Linux from running on that machine
then I'll apply the patch ;)

Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Andrew Morton]

On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:

> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> calculating here will generate an unexpected result. In addition, if
> PAGE_SHIFT > 20, The memory size represented by numentries was already
> integral multiple of 1MB.
> 

If you tell me that you have a machine which has PAGE_SIZE=2MB and this
was the only problem which prevented Linux from running on that machine
then I'll apply the patch ;)

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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
> On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
>
>> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
>> calculating here will generate an unexpected result. In addition, if
>> PAGE_SHIFT > 20, The memory size represented by numentries was already
>> integral multiple of 1MB.
>>
>
> If you tell me that you have a machine which has PAGE_SIZE=2MB and this
> was the only problem which prevented Linux from running on that machine
> then I'll apply the patch ;)
>

Hi Morton:
I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
PAGE_SHIFT in some architecture is very big.
such as the following in "arch/hexagon/include/asm/page.h"
....
#ifdef CONFIG_PAGE_SIZE_256KB
#define PAGE_SHIFT 18
#define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
#endif

#ifdef CONFIG_PAGE_SIZE_1MB
#define PAGE_SHIFT 20
#define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
#endif
.....

Maybe the day of "A 2MB page" is not far. :-) I know it is just a
latent issue. Even if it won't generate a error when PAGE_SIZE == 20,
the calculating here is not necessary. In my mind, compiler would
optimize the calculating at that situation. But it is a little tricky.

In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
won't generate any machine instruction. Just a guarantee.

--
I love linux!!!

Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
> On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
>
>> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
>> calculating here will generate an unexpected result. In addition, if
>> PAGE_SHIFT > 20, The memory size represented by numentries was already
>> integral multiple of 1MB.
>>
>
> If you tell me that you have a machine which has PAGE_SIZE=2MB and this
> was the only problem which prevented Linux from running on that machine
> then I'll apply the patch ;)
>

Hi Morton:
I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
PAGE_SHIFT in some architecture is very big.
such as the following in "arch/hexagon/include/asm/page.h"
....
#ifdef CONFIG_PAGE_SIZE_256KB
#define PAGE_SHIFT 18
#define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
#endif

#ifdef CONFIG_PAGE_SIZE_1MB
#define PAGE_SHIFT 20
#define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
#endif
.....

Maybe the day of "A 2MB page" is not far. :-) I know it is just a
latent issue. Even if it won't generate a error when PAGE_SIZE == 20,
the calculating here is not necessary. In my mind, compiler would
optimize the calculating at that situation. But it is a little tricky.

In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
won't generate any machine instruction. Just a guarantee.

--
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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Andrew Morton]

On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@gmail.com> wrote:

> 2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
> > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
> >
> >> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> >> calculating here will generate an unexpected result. In addition, if
> >> PAGE_SHIFT > 20, The memory size represented by numentries was already
> >> integral multiple of 1MB.
> >>
> >
> > If you tell me that you have a machine which has PAGE_SIZE=2MB and this
> > was the only problem which prevented Linux from running on that machine
> > then I'll apply the patch ;)
> >
> 
> Hi Morton:
> I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
> PAGE_SHIFT in some architecture is very big.
> such as the following in "arch/hexagon/include/asm/page.h"
> ....
> #ifdef CONFIG_PAGE_SIZE_256KB
> #define PAGE_SHIFT 18
> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
> #endif
> 
> #ifdef CONFIG_PAGE_SIZE_1MB
> #define PAGE_SHIFT 20
> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
> #endif
> .....

Good heavens.

> In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
> won't generate any machine instruction. Just a guarantee.

Well the existing code is a bit silly looking.  Why can't we just do

	/* round applicable memory size up to nearest megabyte */
	if (PAGE_SHIFT < 20)
		numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE);

or similar?

Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Andrew Morton]

On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@gmail.com> wrote:

> 2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
> > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
> >
> >> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
> >> calculating here will generate an unexpected result. In addition, if
> >> PAGE_SHIFT > 20, The memory size represented by numentries was already
> >> integral multiple of 1MB.
> >>
> >
> > If you tell me that you have a machine which has PAGE_SIZE=2MB and this
> > was the only problem which prevented Linux from running on that machine
> > then I'll apply the patch ;)
> >
> 
> Hi Morton:
> I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
> PAGE_SHIFT in some architecture is very big.
> such as the following in "arch/hexagon/include/asm/page.h"
> ....
> #ifdef CONFIG_PAGE_SIZE_256KB
> #define PAGE_SHIFT 18
> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
> #endif
> 
> #ifdef CONFIG_PAGE_SIZE_1MB
> #define PAGE_SHIFT 20
> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
> #endif
> .....

Good heavens.

> In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
> won't generate any machine instruction. Just a guarantee.

Well the existing code is a bit silly looking.  Why can't we just do

	/* round applicable memory size up to nearest megabyte */
	if (PAGE_SHIFT < 20)
		numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE);

or similar?

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Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

2013/7/20 Andrew Morton <akpm@linux-foundation.org>:
> On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@gmail.com> wrote:
>
>> 2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
>> > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
>> >
>> >> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
>> >> calculating here will generate an unexpected result. In addition, if
>> >> PAGE_SHIFT > 20, The memory size represented by numentries was already
>> >> integral multiple of 1MB.
>> >>
>> >
>> > If you tell me that you have a machine which has PAGE_SIZE=2MB and this
>> > was the only problem which prevented Linux from running on that machine
>> > then I'll apply the patch ;)
>> >
>>
>> Hi Morton:
>> I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
>> PAGE_SHIFT in some architecture is very big.
>> such as the following in "arch/hexagon/include/asm/page.h"
>> ....
>> #ifdef CONFIG_PAGE_SIZE_256KB
>> #define PAGE_SHIFT 18
>> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
>> #endif
>>
>> #ifdef CONFIG_PAGE_SIZE_1MB
>> #define PAGE_SHIFT 20
>> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
>> #endif
>> .....
>
> Good heavens.
>
>> In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
>> won't generate any machine instruction. Just a guarantee.
>
> Well the existing code is a bit silly looking.  Why can't we just do
>
>         /* round applicable memory size up to nearest megabyte */
>         if (PAGE_SHIFT < 20)
>                 numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE);
>
> or similar?

Great. I have adjusted these code lines, and sent the latest one.

--
I love linux!!!

Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

2013/7/20 Andrew Morton <akpm@linux-foundation.org>:
> On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@gmail.com> wrote:
>
>> 2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
>> > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
>> >
>> >> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
>> >> calculating here will generate an unexpected result. In addition, if
>> >> PAGE_SHIFT > 20, The memory size represented by numentries was already
>> >> integral multiple of 1MB.
>> >>
>> >
>> > If you tell me that you have a machine which has PAGE_SIZE=2MB and this
>> > was the only problem which prevented Linux from running on that machine
>> > then I'll apply the patch ;)
>> >
>>
>> Hi Morton:
>> I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
>> PAGE_SHIFT in some architecture is very big.
>> such as the following in "arch/hexagon/include/asm/page.h"
>> ....
>> #ifdef CONFIG_PAGE_SIZE_256KB
>> #define PAGE_SHIFT 18
>> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
>> #endif
>>
>> #ifdef CONFIG_PAGE_SIZE_1MB
>> #define PAGE_SHIFT 20
>> #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
>> #endif
>> .....
>
> Good heavens.
>
>> In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
>> won't generate any machine instruction. Just a guarantee.
>
> Well the existing code is a bit silly looking.  Why can't we just do
>
>         /* round applicable memory size up to nearest megabyte */
>         if (PAGE_SHIFT < 20)
>                 numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE);
>
> or similar?

Great. I have adjusted these code lines, and sent the latest one.

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[PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry Zhou]

When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
previous calculating here will generate an unexpected result. In
addition, if PAGE_SIZE >= 1MB, The memory size of "numentries" was
already integral multiple of 1MB.

Signed-off-by: Jerry Zhou <uulinux@gmail.com>
---
 mm/page_alloc.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..7c469c6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5745,9 +5745,10 @@ void *__init alloc_large_system_hash(const char *tablename,
 	if (!numentries) {
 		/* round applicable memory size up to nearest megabyte */
 		numentries = nr_kernel_pages;
-		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
-		numentries >>= 20 - PAGE_SHIFT;
-		numentries <<= 20 - PAGE_SHIFT;
+
+		/* It isn't necessary when PAGE_SIZE >= 1MB */
+		if (PAGE_SHIFT < 20)
+			numentries = round_up(numentries, (1<<20)/PAGE_SIZE);
 
 		/* limit to 1 bucket per 2^scale bytes of low memory */
 		if (scale > PAGE_SHIFT)
-- 
1.8.1.5

[PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry Zhou]

When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
previous calculating here will generate an unexpected result. In
addition, if PAGE_SIZE >= 1MB, The memory size of "numentries" was
already integral multiple of 1MB.

Signed-off-by: Jerry Zhou <uulinux@gmail.com>
---
 mm/page_alloc.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..7c469c6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5745,9 +5745,10 @@ void *__init alloc_large_system_hash(const char *tablename,
 	if (!numentries) {
 		/* round applicable memory size up to nearest megabyte */
 		numentries = nr_kernel_pages;
-		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
-		numentries >>= 20 - PAGE_SHIFT;
-		numentries <<= 20 - PAGE_SHIFT;
+
+		/* It isn't necessary when PAGE_SIZE >= 1MB */
+		if (PAGE_SHIFT < 20)
+			numentries = round_up(numentries, (1<<20)/PAGE_SIZE);
 
 		/* limit to 1 bucket per 2^scale bytes of low memory */
 		if (scale > PAGE_SHIFT)
-- 
1.8.1.5

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