[PATCH] mm: Fix a null ptr deref with CONFIG_DEBUG_VM enabled in wp_page_reuse

[Matthew Wilcox]

On Wed, Jul 27, 2022 at 03:14:07PM -0400, Zack Rusin wrote:
> From: Zack Rusin <zackr at vmware.com>
> 
> Write page faults on last references might not have a valid page anymore.
> wp_page_reuse has always dealt with that scenario by making
> sure the page isn't null (or the reference was shared) before doing
> anything with it. Recently added checks in VM_BUG_ON (enabled by the
> CONFIG_DEBUG_VM option) use PageAnon helpers which assume the passed
> page is never null, before making sure there is a valid page to work
> with.
> 
> Move the VM_BUG_ON, which unconditionally uses the page, after the
> code that checks that we have a valid one.

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