linux-kernel - Re: [PATCH v2] mm: make page_mapped_in

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Message-ID: <8dd155b4-cdf5-42b9-9865-862184339fc2@redhat.com>
Date: Wed, 26 Feb 2025 16:53:06 +0100
From: David Hildenbrand <david@...hat.com>
To: Jane Chu <jane.chu@...cle.com>, willy@...radead.org, peterx@...hat.com,
 akpm@...ux-foundation.org, linmiaohe@...wei.com,
 kirill.shutemov@...ux.intel.com, hughd@...gle.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: make page_mapped_in_vma() hugetlb walk aware

On 26.02.25 16:49, David Hildenbrand wrote:
> On 24.02.25 22:14, Jane Chu wrote:
>> When a process consumes a UE in a page, the memory failure handler
>> attempts to collect information for a potential SIGBUS.
>> If the page is an anonymous page, page_mapped_in_vma(page, vma) is
>> invoked in order to
>>     1. retrieve the vaddr from the process' address space,
>>     2. verify that the vaddr is indeed mapped to the poisoned page,
>> where 'page' is the precise small page with UE.
>>
>> It's been observed that when injecting poison to a non-head subpage
>> of an anonymous hugetlb page, no SIGBUS show up; while injecting to
>> the head page produces a SIGBUS. The casue is that, though hugetlb_walk()
>> returns a valid pmd entry (on x86), but check_pte() detects mismatch
>> between the head page per the pmd and the input subpage. Thus the vaddr
>> is considered not mapped to the subpage and the process is not collected
>> for SIGBUS purpose.  This is the calling stack
>>         collect_procs_anon
>>           page_mapped_in_vma
>>             page_vma_mapped_walk
>>               hugetlb_walk
>>                 huge_pte_lock
>>                   check_pte
>>
> 
> Why can't we require callers to never pass in subpages of hugetlb pages,
> and sanity check that this is the case?

To be precise: in collect_procs_anon() pass the head page of the hugetlb 
page, or in page_mapped_in_vma(), use the head page of the folio for 
hugetlb folios.

hugetlb folios are always entirely mapped, adding logic to detect if 
sub-pages are mapped doesn't make too much sense.

-- 
Cheers,

David / dhildenb