Re: [patch] mm, page_alloc: move_freepages should not examine struct page of reserved memory

[Andrew Morton]

On Mon, 12 Aug 2019 20:37:11 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote:

> After commit 907ec5fca3dc ("mm: zero remaining unavailable struct pages"),
> struct page of reserved memory is zeroed.  This causes page->flags to be 0
> and fixes issues related to reading /proc/kpageflags, for example, of
> reserved memory.
> 
> The VM_BUG_ON() in move_freepages_block(), however, assumes that
> page_zone() is meaningful even for reserved memory.  That assumption is no
> longer true after the aforementioned commit.
> 
> There's no reason why move_freepages_block() should be testing the
> legitimacy of page_zone() for reserved memory; its scope is limited only
> to pages on the zone's freelist.
> 
> Note that pfn_valid() can be true for reserved memory: there is a backing
> struct page.  The check for page_to_nid(page) is also buggy but reserved
> memory normally only appears on node 0 so the zeroing doesn't affect this.
> 
> Move the debug checks to after verifying PageBuddy is true.  This isolates
> the scope of the checks to only be for buddy pages which are on the zone's
> freelist which move_freepages_block() is operating on.  In this case, an
> incorrect node or zone is a bug worthy of being warned about (and the
> examination of struct page is acceptable bcause this memory is not
> reserved).

I'm thinking Fixes:907ec5fca3dc and Cc:stable?  But 907ec5fca3dc is
almost a year old, so you were doing something special to trigger this?