Re: [PATCH v2] mm/madvise: avoid skipping pages after splitting large folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/6/26 07:55, Yunhui Cui wrote:
> madvise_inject_error() advances through the requested range using the
> size of the page returned by get_user_pages_fast(). Saving the size
> before error injection is required for hugetlb pages because successful
> soft offlining can dissolve the source huge page.
> 
> That stride is incorrect for non-hugetlb large folios in system memory.
> The memory failure handlers split such a folio and handle only the base
> page for the supplied PFN. Advancing by the pre-split folio size then
> skips the remaining pages in the requested range while madvise() still
> reports success.
> 
> Advance by PAGE_SIZE for non-hugetlb folios in system memory. Retain
> folio_size() for hugetlb and ZONE_DEVICE folios, as compound Device DAX
> folios are handled as a whole.
> 
> Fixes: 19bfbe22f59a ("mm, hugetlb, soft_offline: save compound page order before page migration")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
> ---
>  mm/madvise.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 5a09cc24f04a0..e9d4c3bbc5290 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1455,20 +1455,25 @@ static int madvise_inject_error(struct madvise_behavior *madv_behavior)
>  
>  	for (; start < end; start += size) {
>  		unsigned long pfn;
> +		struct folio *folio;
>  		struct page *page;
>  		int ret;
>  
>  		ret = get_user_pages_fast(start, 1, 0, &page);
>  		if (ret != 1)
>  			return ret;
> +		folio = page_folio(page);
>  		pfn = page_to_pfn(page);
>  
>  		/*
> -		 * When soft offlining hugepages, after migrating the page
> -		 * we dissolve it, therefore in the second loop "page" will
> -		 * no longer be a compound page.
> +		 * Non-hugetlb large folios in system memory are split and only
> +		 * the addressed base page is handled. Hugetlb folios may be
> +		 * dissolved and ZONE_DEVICE folios may be handled as a whole,
> +		 * so save their size before error injection.
>  		 */
> -		size = page_size(compound_head(page));
> +		size = PAGE_SIZE;
> +		if (folio_test_hugetlb(folio) || folio_is_zone_device(folio))
> +			size = folio_size(folio);


We should never ever try deferring "how much has been mapped" from a single PTE.

While this currently works for hugetlb, it's just an anti-pattern to throw
hugetlb checks and similar around.

So this is not the way to fix it.

-- 
Cheers,

David