Re: [PATCH] hwpoison: fix uninitialized warning
From: Wu Fengguang
Date: Tue Sep 15 2009 - 21:09:15 EST
On Wed, Sep 16, 2009 at 08:51:06AM +0800, Hugh Dickins wrote:
>
On 2009年9月16日, 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@xxxxxxxxxxxxx>
>
> > ---
>
> > 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
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