Re: [PATCH] mm/swapfile: fix swapoff vs. software dirty bits

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Thu, 17 Sep 2015 22:31:52 +0300
Cyrill Gorcunov <gorcunov@gmail.com> wrote:

> Thus when CONFIG_MEM_SOFT_DIRTY = n, the unuse_pte will be the same
> as it were without the patch, calling pte_same.
> 
> Now to the bit itself
> 
> #ifdef CONFIG_MEM_SOFT_DIRTY
> #define _PAGE_SWP_SOFT_DIRTY_PAGE_PSE
> #else
> #define _PAGE_SWP_SOFT_DIRTY_PAGE_PSE(_AT(pteval_t, 0))
> #endif
>
> it's 0 if CONFIG_MEM_SOFT_DIRTY=n, so any setup of this
> bit will simply become nop

Ok, that is what I have been missing with my soft dirty patch for s390.
_PAGE_BIT_SOFT_DIRTY is always defined but the _PAGE_SOFT_DIRTY and
_PAGE_SWP_SOFT_DIRTY are conditional. The primitives are always
defined but turn into nops with CONFIG_MEM_SOFT_DIRTY=n.

> #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> {
> 	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
> }
> 
> static inline int pte_swp_soft_dirty(pte_t pte)
> {
> 	return pte_flags(pte) & _PAGE_SWP_SOFT_DIRTY;
> }
> 
> static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
> {
> 	return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
> }
> #endif
> 
> So I fear I'm lost where this "set" of the bit comes from
> when CONFIG_MEM_SOFT_DIRTY=n.
> 
> Martin, could you please elaborate? Seems I'm missing
> something obvious.
 
It is me who missed something.. thanks for the explanation.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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