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Subject: Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for
 compound pages
From:   John Hubbard <jhubbard@nvidia.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     "Kirill A. Shutemov" <kirill@shutemov.name>,
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On 1/29/20 10:23 PM, John Hubbard wrote:
> On 1/29/20 2:59 PM, Matthew Wilcox wrote:
> ...
>> I have a hunk in my current tree which looks like this:
>>
>> @@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reaso=
n)
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0 pr_warn("page:%px refcount:%d mapcount:%d mapping:=
%px index:%#lx
>> \n",
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 pa=
ge, page_ref_count(page), mapcount,
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 pa=
ge->mapping, page_to_pgoff(page));
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (PageTail(page)) {
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0 struct page *head =3D compound_head(page);
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0 pr_warn("head:%px mapping:%px index:%#lx\n",
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 head, head-=
>mapping, page_to_pgoff(head));
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (PageKsm(page))
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0 pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &p=
age->flags);
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 else if (PageAnon(page)=
)
>>
>> I wonder if we can combine these two patches in some more useful way?
>>
>> I also think we probably want a sanity check that 'head' and 'page'
>> are within a sane range of each other (ie head < page and head +
>> MAX_ORDER_NR_PAGES > page) to protect against a struct page that contain=
s
>> complete garbage.
>>
>=20
> OK, here's a go at combining those. I like the observation, implicit in y=
our
> diffs, that PageTail rather than PageCompound is the key differentiator i=
n
> deciding what to print. How's this look:
>=20
> diff --git a/mm/debug.c b/mm/debug.c
> index a90da5337c14..944652843e7b 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -75,12 +75,31 @@ void __dump_page(struct page *page, const char *reaso=
n)
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */
>  =C2=A0=C2=A0=C2=A0=C2=A0 mapcount =3D PageSlab(page) ? 0 : page_mapcount=
(page);
>=20
> -=C2=A0=C2=A0=C2=A0 if (PageCompound(page))
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 pr_warn("page:%px refcount:%d=
 mapcount:%d mapping:%px "
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "inde=
x:%#lx compound_mapcount: %d\n",
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 page,=
 page_ref_count(page), mapcount,
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 page-=
>mapping, page_to_pgoff(page),
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 compo=
und_mapcount(page));
> +=C2=A0=C2=A0=C2=A0 if (PageTail(page)) {
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct page *head =3D compoun=
d_head(page);
> +
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if ((page < head) || (page >=
=3D head + MAX_ORDER_NR_PAGES)) {
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /*
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
 * Page is hopelessly corrupted, so limit any reporting
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
 * to information about the page itself. Do not attempt
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
 * to look at the head page.
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
 */
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 pr_wa=
rn("page:%px refcount:%d mapcount:%d mapping:%px "
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 "index:%#lx (corrupted tail page case)\n",
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 page, page_ref_count(page), mapcount,
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 page->mapping, page_to_pgoff(page));
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } else {
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 pr_wa=
rn("page:%px compound refcount:%d mapcount:%d "
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 "mapping:%px index:%#lx compound_mapcount:%d\n",
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 page, page_ref_count(head),
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 mapcount, head->mapping, page_to_pgoff(head),
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 compound_mapcount(page));
> +
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (p=
age_ref_count(page) !=3D 0)
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 pr_warn("page:%px PROBLEM: non-zero refcount (=3D=3D%d) =
on "
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "this tail page\n", page, page_r=
ef_count(page));

...ahem, I sorta botched the above statement, because that should
be outside (just below) the "else" statement--it can be done whether or
not the page fails the safety/bounds check. :)

thanks,
--=20
John Hubbard
NVIDIA