[PATCH] hwpoison: fix uninitialized warning

[PATCH] hwpoison: fix uninitialized warning

[Hugh Dickins]

Fix mmotm build warning, presumably also in linux-next:
mm/memory.c: In function `do_swap_page':
mm/memory.c:2498: warning: `pte' may be used uninitialized in this function

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---
I've only noticed this warning on one machine, the powerpc: certainly it
needs CONFIG_MIGRATION or CONFIG_MEMORY_FAILURE to see it, but I thought
I had one of those set on other machines - just musing in case it's being
masked elsewhere by some other bug...

 mm/memory.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- mmotm/mm/memory.c	2009-09-14 16:34:37.000000000 +0100
+++ linux/mm/memory.c	2009-09-15 22:00:48.000000000 +0100
@@ -2495,7 +2495,7 @@ static int do_swap_page(struct mm_struct
 		} else if (is_hwpoison_entry(entry)) {
 			ret = VM_FAULT_HWPOISON;
 		} else {
-			print_bad_pte(vma, address, pte, NULL);
+			print_bad_pte(vma, address, orig_pte, NULL);
 			ret = VM_FAULT_OOM;
 		}
 		goto out;

Re: [PATCH] hwpoison: fix uninitialized warning

[Wu Fengguang]

On Wed, Sep 16, 2009 at 05:19:07AM +0800, Hugh Dickins wrote:
> Fix mmotm build warning, presumably also in linux-next:
> mm/memory.c: In function `do_swap_page':
> mm/memory.c:2498: warning: `pte' may be used uninitialized in this function

Thanks for the fix!

Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> ---
> I've only noticed this warning on one machine, the powerpc: certainly it
> needs CONFIG_MIGRATION or CONFIG_MEMORY_FAILURE to see it, but I thought
> I had one of those set on other machines - just musing in case it's being
> masked elsewhere by some other bug...
> 
>  mm/memory.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- mmotm/mm/memory.c	2009-09-14 16:34:37.000000000 +0100
> +++ linux/mm/memory.c	2009-09-15 22:00:48.000000000 +0100
> @@ -2495,7 +2495,7 @@ static int do_swap_page(struct mm_struct
>  		} else if (is_hwpoison_entry(entry)) {
>  			ret = VM_FAULT_HWPOISON;
>  		} else {
> -			print_bad_pte(vma, address, pte, NULL);
> +			print_bad_pte(vma, address, orig_pte, NULL);
>  			ret = VM_FAULT_OOM;
>  		}
>  		goto out;

Re: [PATCH] hwpoison: fix uninitialized warning

[Minchan Kim]

On Wed, Sep 16, 2009 at 6:19 AM, Hugh Dickins
<hugh.dickins@tiscali.co.uk> wrote:
> Fix mmotm build warning, presumably also in linux-next:
> mm/memory.c: In function `do_swap_page':
> mm/memory.c:2498: warning: `pte' may be used uninitialized in this function
>
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

-- 
Kind regards,
Minchan Kim

Re: [PATCH] hwpoison: fix uninitialized warning

[Wu Fengguang]

On Wed, Sep 16, 2009 at 05:19:07AM +0800, Hugh Dickins wrote:
> Fix mmotm build warning, presumably also in linux-next:
> mm/memory.c: In function `do_swap_page':
> mm/memory.c:2498: warning: `pte' may be used uninitialized in this function
> 
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> ---
> I've only noticed this warning on one machine, the powerpc: certainly it
> needs CONFIG_MIGRATION or CONFIG_MEMORY_FAILURE to see it, but I thought
> I had one of those set on other machines - just musing in case it's being
> masked elsewhere by some other bug...
> 
>  mm/memory.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- mmotm/mm/memory.c	2009-09-14 16:34:37.000000000 +0100
> +++ linux/mm/memory.c	2009-09-15 22:00:48.000000000 +0100
> @@ -2495,7 +2495,7 @@ static int do_swap_page(struct mm_struct
>  		} else if (is_hwpoison_entry(entry)) {
>  			ret = VM_FAULT_HWPOISON;
>  		} else {
> -			print_bad_pte(vma, address, pte, NULL);
> +			print_bad_pte(vma, address, orig_pte, NULL);
>  			ret = VM_FAULT_OOM;
>  		}

The lines was introduced in this patch:

        entry = pte_to_swp_entry(orig_pte);                                                                                          
-       if (is_migration_entry(entry)) {                                                                                             
-               migration_entry_wait(mm, pmd, address);                                                                              
+       if (unlikely(non_swap_entry(entry))) {                                                                                       
+               if (is_migration_entry(entry)) {                                                                                     
+                       migration_entry_wait(mm, pmd, address);                                                                      
+               } else if (is_hwpoison_entry(entry)) {                                                                               
+                       ret = VM_FAULT_HWPOISON;                                                                                     
+               } else {                                                                                                             
+                       print_bad_pte(vma, address, pte, NULL);                                                                      
+                       ret = VM_FAULT_OOM;                                                                                          
+               }                                                                                                                    
                goto out;                                                                                                            
        }                                                                                                                            

Given that currently there are only two types of non swap entries:
migration/hwpoison, the last 'else' block is in fact dead code..

Thanks,
Fengguang

Re: [PATCH] hwpoison: fix uninitialized warning

[Hugh Dickins]

On Wed, 16 Sep 2009, Wu Fengguang wrote:
> On Wed, Sep 16, 2009 at 05:19:07AM +0800, Hugh Dickins wrote:
> > Fix mmotm build warning, presumably also in linux-next:
> > mm/memory.c: In function `do_swap_page':
> > mm/memory.c:2498: warning: `pte' may be used uninitialized in this function
> > 
> > Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> > ---
> > I've only noticed this warning on one machine, the powerpc: certainly it
> > needs CONFIG_MIGRATION or CONFIG_MEMORY_FAILURE to see it, but I thought
> > I had one of those set on other machines - just musing in case it's being
> > masked elsewhere by some other bug...

> The lines was introduced in this patch:
> 
>         entry = pte_to_swp_entry(orig_pte);
> -       if (is_migration_entry(entry)) {
> -               migration_entry_wait(mm, pmd, address);
> +       if (unlikely(non_swap_entry(entry))) {
> +               if (is_migration_entry(entry)) {
> +                       migration_entry_wait(mm, pmd, address);
> +               } else if (is_hwpoison_entry(entry)) {
> +                       ret = VM_FAULT_HWPOISON;
> +               } else {
> +                       print_bad_pte(vma, address, pte, NULL);
> +                       ret = VM_FAULT_OOM;
> +               }
>                 goto out;
>         }
> 
> Given that currently there are only two types of non swap entries:
> migration/hwpoison, the last 'else' block is in fact dead code..

Ah, yes, I think it is dead code on x86 (32 and 64), where the
swp_entry_t is well packed.  But not dead code on ppc64, which has

#define __swp_type(entry)	(((entry).val >> 1) & 0x3f)

which is allowing swap types up to 63, when in fact the highest
we use is 31: that leaves space for 32 more non_swap_entry types.

So the compiler was absolutely right to complain about the
uninitialized variable on ppc64, but not on x86.  It's a little
surprising that ppc64 allows 64 swap types, but nothing wrong.

Thanks,
Hugh

Re: [PATCH] hwpoison: fix uninitialized warning

[Wu Fengguang]

On Wed, Sep 16, 2009 at 08:51:06AM +0800, Hugh Dickins wrote:
> On Wed, 16 Sep 2009, Wu Fengguang wrote:
> > On Wed, Sep 16, 2009 at 05:19:07AM +0800, Hugh Dickins wrote:
> > > Fix mmotm build warning, presumably also in linux-next:
> > > mm/memory.c: In function `do_swap_page':
> > > mm/memory.c:2498: warning: `pte' may be used uninitialized in this function
> > > 
> > > Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> > > ---
> > > I've only noticed this warning on one machine, the powerpc: certainly it
> > > needs CONFIG_MIGRATION or CONFIG_MEMORY_FAILURE to see it, but I thought
> > > I had one of those set on other machines - just musing in case it's being
> > > masked elsewhere by some other bug...
> 
> > The lines was introduced in this patch:
> > 
> >         entry = pte_to_swp_entry(orig_pte);
> > -       if (is_migration_entry(entry)) {
> > -               migration_entry_wait(mm, pmd, address);
> > +       if (unlikely(non_swap_entry(entry))) {
> > +               if (is_migration_entry(entry)) {
> > +                       migration_entry_wait(mm, pmd, address);
> > +               } else if (is_hwpoison_entry(entry)) {
> > +                       ret = VM_FAULT_HWPOISON;
> > +               } else {
> > +                       print_bad_pte(vma, address, pte, NULL);
> > +                       ret = VM_FAULT_OOM;
> > +               }
> >                 goto out;
> >         }
> > 
> > Given that currently there are only two types of non swap entries:
> > migration/hwpoison, the last 'else' block is in fact dead code..
> 
> Ah, yes, I think it is dead code on x86 (32 and 64), where the
> swp_entry_t is well packed.  But not dead code on ppc64, which has
> 
> #define __swp_type(entry)	(((entry).val >> 1) & 0x3f)
> 
> which is allowing swap types up to 63, when in fact the highest
> we use is 31: that leaves space for 32 more non_swap_entry types.
> 
> So the compiler was absolutely right to complain about the
> uninitialized variable on ppc64, but not on x86.  It's a little
> surprising that ppc64 allows 64 swap types, but nothing wrong.

Ah I know. It seems that gcc is smart enough to remove that dead code
and hence the warning message in x86 :)

Thanks,
Fengguang