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Date:   Mon, 12 Jul 2021 08:53:13 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Chao Yu <chao@kernel.org>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Matthew Wilcox <willy@infradead.org>,
        Jaegeuk Kim <jaegeuk@kernel.org>, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for
 our internal use
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On Sat 10-07-21 16:11:38, Chao Yu wrote:
> On 2021/7/7 17:57, Mel Gorman wrote:
> > I think it would work but it would be preferable to find out why the
> > tail page has an order set in the first place. I've looked over
> 
> Agreed.
> 
> > mm/page_alloc.c and mm/compaction.c a few times and did not spot where
> > set_private_page(page, 0) is missed when it should be covered by
> > clear_page_guard or del_page_from_free_list :(
> 
> I didn't enable CONFIG_DEBUG_PAGEALLOC, so we will expect page private
> should be cleared by del_page_from_free_list(), but I guess it only clears
> the buddy's private field rather than original page's, so I added below
> diff and check the dmesg, it looks stall private value in original page
> will be left commonly... Let me know if I missed something?

Page private should be cleared when the page is freed to the allocator.
Have a look at PAGE_FLAGS_CHECK_AT_FREE.

> ---
>  mm/page_alloc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a06bcfe6f786..1e7031ff548e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1029,6 +1029,7 @@ static inline void __free_one_page(struct page *page,
>  	unsigned long combined_pfn;
>  	unsigned int max_order;
>  	struct page *buddy;
> +	struct page *orig_page = page;
>  	bool to_tail;
> 
>  	max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);
> @@ -1097,6 +1098,10 @@ static inline void __free_one_page(struct page *page,
> 
>  done_merging:
>  	set_buddy_order(page, order);
> +	if (orig_page != page) {
> +		if (WARN_ON_ONCE(orig_page->private))
> +			pr_info("2order:%x, origpage.private:%x", order, orig_page->private);
> +	}

Why is this expected? Buddy allocator uses page private to store order.
Whether we are merging to the freed page or coalesce it to a different
page is not all that important.
-- 
Michal Hocko
SUSE Labs