Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page

[Andrea Arcangeli]

On Tue, Apr 14, 2009 at 03:16:52PM +0900, KOSAKI Motohiro wrote:
> +	if (PageSwapCache(page) &&
> +	    page_count(page) != page_mapcount(page) + 2) {
> +		ret = SWAP_FAIL;
> +		goto out_unmap;
> +	}
> +

Besides the race pointed out by Nick, this also would break KVM
swapping with mmu notifier. mmu_notifier_invalidate_page must be
invoked before reading page_count for this to work. However the
invalidate has to be moved below the
mlock/ptep_clear_flush_young_notify, no point to get rid of sptes if
any of the spte or the pte is still young.
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