Re: [RFC PATCH 1/5] mm/page_alloc: count guard pages as free again when their buddy merges

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Oct 06, 2026 at 10:12:46PM -0400, Rik van Riel wrote:
> With debug_guardpage_minorder set, expand() turns the unused halves of a
> split into guard pages, which page_del_and_expand() leaves out of the free
> counts. Freeing the allocated half credits only that half, and
> __free_one_page() then merges the guard buddy with clear_page_guard(),
> which no longer touches the counters.
> 
> The merged block goes on a free list with the guard pages never counted,
> so NR_FREE_PAGES falls behind the free lists by every guard that merges.
> Before commit e0932b6c1f94 ("mm: page_alloc: consolidate free page
> accounting"), __set_page_guard() and __clear_page_guard() adjusted the
> counts themselves; that commit dropped both adjustments but kept the
> subtraction implicit in expand().
> 
> Count the guard pages under the merged block's migratetype when the
> buddy is cleared. account_freepages() skips an isolated block as it does
> for every other free page, and moving the block off the isolated list
> counts it then.
> 
> Booting a 16GB VM with debug_pagealloc=on debug_guardpage_minorder=1,
> check the difference in free pages reported between /proc/vmstat
> and /proc/buddyinfo.
> 
> At three points (idle, after a read and file-creation load, after
> a second load), check the difference between nr_free_pages and the
> buddyinfo numbers in the normal zone, as a number of pages:
> 
>                 idle   after load   after second load
>     unpatched   -130        79205               99121
>     patched      -16           45                   0
> 
> The small remaining difference seems to be due to the numbers
> not being read at exactly the same time, and is also seen
> without guard pages.
> 
> Fixes: e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Rik van Riel <riel@surriel.com>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>

This should probably be not RFC?