Re: [PATCH v2] mm: make page_mapped_in_vma() hugetlb walk aware

[jane.chu@oracle.com]

On 2/26/2025 7:49 AM, 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?

Because for memory-failure handling, we want to pin point to the exact 
small page even if the small page is part of a huge page, so that if 
userspace could manage to recover, they don't have to recover the clean 
subpages.  Please refer to 376907f3a0b34 ("mm/memory-failure: pass the 
folio and the page to collect_procs()") for the change.

thanks,

-jane

>
>