Re: [PATCH v2 2/5] mm/rmap: Add try_to_unmap_hugetlb_one

[Dev Jain <dev.jain@arm.com>]

On 13/07/26 4:04 pm, David Hildenbrand (Arm) wrote:
> On 7/10/26 15:16, Dev Jain wrote:
>>
>>
>> On 10/07/26 11:29 am, Dev Jain wrote:
>>> Simplify try_to_unmap_one() by separating the hugetlb parts into
>>> try_to_unmap_hugetlb_one().
>>>
>>> To understand the correctness of the refactoring, the following points
>>> are noted:
>>>
>>> 1. try_to_unmap() is called for hugetlb folios only when they are
>>>    hwpoisoned.
>>>
>>> 2. A hugetlb VMA cannot be mlocked.
>>>
>>> 3. page_vma_mapped_walk() returns at most one hugetlb mapping in a VMA,
>>>    and that mapping points at the head PFN.
>>>
>>> 4. We won't ever process a softleaf entry that encodes a hugetlb folio;
>>>    hugetlb folios are never swapped out, migration entries will be
>>>    skipped (PVMW_MIGRATION not passed), and device-exclusive does not
>>>    work for hugetlb.
>>>
>>> 5. The hwpoison entry is constructed from the poisoned folio, just as in
>>>    the pre-refactor code. Any previous uffd-wp state is deliberately not
>>>    preserved for the hwpoison entry.
>>>
>>> 6. TTU_HWPOISON is always present; for it to not be present, either the
>>>    folio has to be in swapcache, or mapping_can_writeback() is true (see
>>>    unmap_poisoned_folio), none of which is true for hugetlb folios.
>>>
>>> 7. Hugetlb uses separate counters from normal rss counters, therefore
>>>    update_highwater_rss() need not be called.
>>>
>>> While at it:
>>>
>>>  - Change VM_BUG_* to VM_WARN_*.
>>>
>>>  - Do not declare variables which are only used once.
>>>
>>>  - Constify some variables.
>>>
>>>  - Add some more VM_WARN_* to assert some invariants.
>>>
>>> Except the above 4 points, no functional change intended.
>>>
>>> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
>>> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>> ---
>>
>> Andrew, could you squash the following diff:
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index cf02b315a2ead..b61051b879c4c 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -2229,8 +2229,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>                 update_hiwater_rss(mm);
>>
>>                 /*
>> -                * With TTU_HWPOISON, we only expect small folios or hugetlb
>> -                * folios here for now.
>> +                * With TTU_HWPOISON, we only expect small folios here.
>>                  */
>>                 if (folio_test_hwpoison(folio) && (flags & TTU_HWPOISON)) {
>>                         pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
> 
> Note that the comment likely fits into a single line now.

Yes, took care of that in v3 :)


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