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Date:   Mon, 16 Oct 2023 09:32:00 +0900
From:   Hyesoo Yu <hyesoo.yu@samsung.com>
To:     Vishal Moola <vishal.moola@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: page_alloc: check the order of compound page event
 when the order is 0
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On Fri, Oct 13, 2023 at 01:54:08PM -0700, Vishal Moola wrote:
> On Thu, Oct 12, 2023 at 10:11:06AM +0900, Hyesoo Yu wrote:
> > For compound pages, the head sets the PG_head flag and
> > the tail sets the compound_head to indicate the head page.
> > If a user allocates a compound page and frees it with a different
> > order, the compound page information will not be properly
> > initialized. To detect this problem, compound_page(page) and
> > the order are compared, but it is not checked when the order is 0.
> > That error should be checked regardless of the order.
> 
> I believe all compound pages are order >= 1, so this error can't occur
> when the order is 0.
> 

Yes. All compound pages are order >= 1.
However if the user uses the API incorrectly, the order value could be zero.

For example,

addr = alloc_pages(GFP_COMP, 2);
free_pages(addr, 0);

(struct page[16])0xFFFFFFFE21715100 = (
(flags = 0x4000000000000200, lru = (next = 0x0, prev = 0xDEAD000000000122),//  Clear PG_head
(flags = 0x4000000000000000, lru = (next = 0xFFFFFFFE21715101, prev = 0xFFFFFFFF00000201),  // Remain compound head

It is memory leak, and it also makes system stability problem.
on isolation_single_pageblock, That case makes infinite loops.

for (pfn = start_pfn; pfn < boundary_pfn; ) {
	if (PageCompound(page)) { // page[1] is compound page
		struct page *head = compound_head(page); // page[0]
		unsigned long head_pfn = page_to_pfn(head);
		unsigned long nr_pages = compound_nr(head); // nr_pages is 1 since page[0] is not compound page.

 		if (head_pfn + nr_pages <= boundary_pfn) {
			pfn = head_pfn + nr_pages; // pfn is set as page[1].
			continue;
		}
}

So, I guess, we have to check the incorrect use in free_pages_prepare.

Thanks,
Hyesoo Yu.

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