Re: [PATCH v2 1/5] mm/rmap: convert page -> folio for hwpoison checks

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

On 23/07/26 10:45 pm, Lorenzo Stoakes (ARM) wrote:
> On Fri, Jul 10, 2026 at 05:59:10AM +0000, Dev Jain wrote:
>> try_to_unmap() receives hugetlb folios only from the hwpoison path.
> 
> Horrifying :)
> 
>> hugetlb_update_hwpoison() sets the hugetlb folio's head-page
>> hwpoison bit, and page_vma_mapped_walk() reports the hugetlb mapping at
>> the head PFN, so the previous PageHWPoison(subpage) check happened to
>> work for hugetlb.
>>
>> For non-hugetlb folios, unmap_poisoned_folio() currently rejects large
>> folios before calling try_to_unmap(). Hence it is always the case that
>> if try_to_unmap_one() handles an hwpoisoned folio, then the head page is
>> marked with the poison bit.
>>
>> Therefore, convert the poisoned subpage checks to folio_test_hwpoison().
>>
>> No functional change intended, except that, while at it,
>> convert VM_BUG_* to VM_WARN_*.
>>
>> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
> 
> Small nit below but generally LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> 
>> ---
>>  mm/rmap.c | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 9939400e77c79..dbb077f8443e3 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -2122,10 +2122,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>  			bool anon = folio_test_anon(folio);
>>
>>  			/*
>> -			 * The try_to_unmap() is only passed a hugetlb page
>> -			 * in the case where the hugetlb page is poisoned.
>> +			 * The try_to_unmap() is only passed a hugetlb folio
>> +			 * in the case where the hugetlb folio contains a
>> +			 * poisoned page.
>>  			 */
>> -			VM_BUG_ON_PAGE(!PageHWPoison(subpage), subpage);
>> +			VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
>>  			/*
>>  			 * huge_pmd_unshare may unmap an entire PMD page.
>>  			 * There is no way of knowing exactly which PMDs may
>> @@ -2204,7 +2205,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>  		/* Update high watermark before we lower rss */
>>  		update_hiwater_rss(mm);
>>
>> -		if (PageHWPoison(subpage) && (flags & TTU_HWPOISON)) {
>> +		/*
>> +		 * With TTU_HWPOISON, we only expect small folios or hugetlb
>> +		 * folios here for now.
>> +		 */
> 
> I think probably we can drop this? It seems implied by the code (David comment
> similarly I think? :)

I had posted a v3 here:
https://lore.kernel.org/all/20260713050050.1017741-1-dev.jain@arm.com/

In that I had fixed this comment to drop "or hugetlb folios ..." . But I would
still like to retain the "only expect small folios" comment, it adds cognitive
load when I am reading try_to_unmap_one and then have to go read hwpoison code
to figure that large folios would be split before coming here.


> 
>> +		if (folio_test_hwpoison(folio) && (flags & TTU_HWPOISON)) {
>>  			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
>>  			if (folio_test_hugetlb(folio)) {
>>  				hugetlb_count_sub(folio_nr_pages(folio), mm);
>> --
>> 2.43.0
>>
> 
> Cheers, Lorenzo