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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/16/26 18:23, 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 > Reviewed-by: Zi Yan > Reviewed-by: Johannes Weiner > Reviewed-by: Dev Jain Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/page_alloc.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 12fac9084c483..4658af97be010 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -988,10 +988,12 @@ static inline void __free_one_page(struct page *page, > * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page, > * merge with it and move up one order. > */ > - if (page_is_guard(buddy)) > + if (page_is_guard(buddy)) { > clear_page_guard(zone, buddy, order); > - else > + account_freepages(zone, 1 << order, migratetype); > + } else { > __del_page_from_free_list(buddy, zone, order, buddy_mt); > + } > > if (unlikely(buddy_mt != migratetype)) { > /* >