linux-kernel - Re: [PATCH v2 6/9] mm/memory: convert print_bad_pte() to print_bad_page

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Message-ID: <9001b918-a08d-4b38-9737-b5d9949668be@redhat.com>
Date: Fri, 18 Jul 2025 10:26:45 +0200
From: David Hildenbrand <david@...hat.com>
To: Demi Marie Obenour <demiobenour@...il.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, xen-devel@...ts.xenproject.org,
 linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
 Andrew Morton <akpm@...ux-foundation.org>, 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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "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 v2 6/9] mm/memory: convert print_bad_pte() to
 print_bad_page_map()

On 18.07.25 09:59, Demi Marie Obenour wrote:
> On 7/18/25 03:44, David Hildenbrand wrote:
>> On 18.07.25 00:06, Demi Marie Obenour wrote:
>>> On 7/17/25 07:52, 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 -- we'll rename it to what
>>>> we actually print: bad (page) mappings. Maybe it should be called
>>>> "print_bad_table_entry()"? We'll just call it "print_bad_page_map()"
>>>> because the assumption is that we are dealing with some (previously)
>>>> present page table entry that got corrupted in weird ways.
>>>>
>>>> Whether it is a PTE or something else will usually become obvious from the
>>>> page table dump or from the dumped stack. If ever required in the future,
>>>> we could pass the entry level type similar to "enum rmap_level". For now,
>>>> let's keep it simple.
>>>>
>>>> To make the function a bit more readable, factor out the ratelimit check
>>>> into is_bad_page_map_ratelimited() and place the dumping of page
>>>> table content into __dump_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.
>>>>
>>>> Use print_bad_page_map() in vm_normal_page_pmd() similar to how we do it
>>>> for vm_normal_page(), now that we have a function that can handle it.
>>>>
>>>> The report will now look something like (dumping pgd to pmd values):
>>>>
>>>> [   77.943408] BUG: Bad page map in process XXX  entry: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>
>>>
>>> Should this still use a WARN?  If the admin sets panic-on-warn they
>>> have asked for "crash if anything goes wrong" and so that is what
>>> they should get.  Otherwise the system will still stay up.
>>
>> I assume you're comment is in context of the other proposal regarding
>> panicking.
> 
> Which one?  I'm only subscribed to xen-devel and might have missed it.

This one here:

https://lkml.kernel.org/r/4e1b7d2d-ed54-4e0a-a0a4-906b14d9cd41@p183

-- 
Cheers,

David / dhildenb