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Date:   Mon, 16 Oct 2023 16:23:05 +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 Sun, Oct 15, 2023 at 08:28:18PM -0700, Vishal Moola wrote:
> On Sun, Oct 15, 2023 at 5:42 PM Hyesoo Yu <hyesoo.yu@samsung.com> wrote:
> >
> > 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
> 
> s/compound_page/compound_order/
> 
> > > > the order are compared, but it is not checked when the order is 0.
> > > > That error should be checked regardless of the order.
> 
> With this many mentions of "the order", it is easy to misinterpret "the
> order"
> to be referencing the page order rather than the order of pages we are
> trying
> to free. I recommend replacing "the order" with "the order argument" or
> something similar for clarity.
> 

What a good idea! I'll replace that. Thanks for your comments.

> > > 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.
> 
> I see, thanks for clarifying that.
> 
> With the commit message changes above:
> Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> 

Okay, Thanks for review.

Regards.
Hyesoo Yu.

> > 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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