linux-kernel - Re: [PATCH v3 08/11] mm/memory: convert print_bad_pte() to print_bad_page

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Message-ID: <9ad69d4f-69b7-4998-9639-4e8f679a2d9d@lucifer.local>
Date: Tue, 26 Aug 2025 06:25:27 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, linux-fsdevel@...r.kernel.org,
        nvdimm@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Hugh Dickins <hughd@...gle.com>, Oscar Salvador <osalvador@...e.de>,
        Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v3 08/11] mm/memory: convert print_bad_pte() to
 print_bad_page_map()

On Mon, Aug 25, 2025 at 02:31:00PM +0200, David Hildenbrand wrote:
> On 11.08.25 13:26, David Hildenbrand wrote:
> > print_bad_pte() looks like something that should actually be a WARN
> > or similar, but historically it apparently has proven to be useful to
> > detect corruption of page tables even on production systems -- report
> > the issue and keep the system running to make it easier to actually detect
> > what is going wrong (e.g., multiple such messages might shed a light).
> >
> > As we want to unify vm_normal_page_*() handling for PTE/PMD/PUD, we'll have
> > to take care of print_bad_pte() as well.
> >
> > Let's prepare for using print_bad_pte() also for non-PTEs by adjusting the
> > implementation and renaming the function to print_bad_page_map().
> > Provide print_bad_pte() as a simple wrapper.
> >
> > Document the implicit locking requirements for the page table re-walk.
> >
> > To make the function a bit more readable, factor out the ratelimit check
> > into is_bad_page_map_ratelimited() and place the printing of page
> > table content into __print_bad_page_map_pgtable(). We'll now dump
> > information from each level in a single line, and just stop the table
> > walk once we hit something that is not a present page table.
> >
> > The report will now look something like (dumping pgd to pmd values):
> >
> > [   77.943408] BUG: Bad page map in process XXX  pte:80000001233f5867
> > [   77.944077] addr:00007fd84bb1c000 vm_flags:08100071 anon_vma: ...
> > [   77.945186] pgd:10a89f067 p4d:10a89f067 pud:10e5a2067 pmd:105327067
> >
> > Not using pgdp_get(), because that does not work properly on some arm
> > configs where pgd_t is an array. Note that we are dumping all levels
> > even when levels are folded for simplicity.
> >
> > Signed-off-by: David Hildenbrand <david@...hat.com>
> > ---
> >   include/linux/pgtable.h |  19 ++++++++
> >   mm/memory.c             | 104 ++++++++++++++++++++++++++++++++--------
> >   2 files changed, 103 insertions(+), 20 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index bff5c4241bf2e..33c84b38b7ec6 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1966,6 +1966,25 @@ enum pgtable_level {
> >   	PGTABLE_LEVEL_PGD,
> >   };
> > +static inline const char *pgtable_level_to_str(enum pgtable_level level)
> > +{
> > +	switch (level) {
> > +	case PGTABLE_LEVEL_PTE:
> > +		return "pte";
> > +	case PGTABLE_LEVEL_PMD:
> > +		return "pmd";
> > +	case PGTABLE_LEVEL_PUD:
> > +		return "pud";
> > +	case PGTABLE_LEVEL_P4D:
> > +		return "p4d";
> > +	case PGTABLE_LEVEL_PGD:
> > +		return "pgd";
> > +	default:
> > +		VM_WARN_ON_ONCE(1);
> > +		return "unknown";
> > +	}
> > +}
>
> One kernel config doesn't like the VM_WARN_ON_ONCE here, and I don't think we
> really need it. @Andrew can you squash:

Out of interest do you know why this is happening? xtensa right? Does
xtensa not like CONFIG_DEBUG_VM?