Re: [PATCH -mm][BUGFIX] migration_entry_wait fix. v2

[KOSAKI Motohiro]

> In speculative page cache look up protocol, page_count(page) is set to 0
> while radix-tree modification is going on, truncation, migration, etc...
> 
> While page migration, a page fault to page under migration does
>  - look up page table
>  - find it is migration_entry_pte
>  - decode pfn from migration_entry_pte and get page of pfn_page(pfn)
>  - wait until page is unlocked 
> 
> It does get_page() -> wait_on_page_locked() -> put_page() now.
> 
> In page migration's radix-tree replacement, page_freeze_refs() ->
> page_unfreeze_refs() is called. And page_count(page) turns to be zero
> and must be kept to be zero while radix-tree replacement.
> 
> If get_page() is called against a page under radix-tree replacement,
> the kernel panics(). To avoid this, we shouldn't increment page_count()
> if it is zero. This patch uses get_page_unless_zero().
> 
> Even if get_page_unless_zero() fails, the caller just retries.
> But will be a bit busier.

Great!
	Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>


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