Re: [PATCH v2] khugepaged: remove redundant index check for pmd-folios

[Dev Jain]

On 04/03/26 1:57 pm, Wei Yang wrote:
> On Fri, Feb 27, 2026 at 08:05:01PM +0530, Dev Jain wrote:
>> Claim: folio_order(folio) == HPAGE_PMD_ORDER => folio->index == start.
>>
>> Proof: Both loops in hpage_collapse_scan_file and collapse_file, which
>> iterate on the xarray, have the invariant that
>> start <= folio->index < start + HPAGE_PMD_NR ... (i)
>>
>> A folio is always naturally aligned in the pagecache, therefore
>> folio_order == HPAGE_PMD_ORDER => IS_ALIGNED(folio->index, HPAGE_PMD_NR) == true ... (ii)
> 
> This is because __filemap_add_folio() align the index to folio_order(), right?

No, see code around instances of mapping_align_index(). We retrieve the max
order from the index, not the other way around. We already are given the
index to put the folio at, by the caller, so have to construct the order
from that.

> 
>>
>> thp_vma_allowable_order -> thp_vma_suitable_order requires that the virtual
>> offsets in the VMA are aligned to the order,
>> => IS_ALIGNED(start, HPAGE_PMD_NR) == true ... (iii)
>>
>> Combining (i), (ii) and (iii), the claim is proven.
>>
>> Therefore, remove this check.
>> While at it, simplify the comments.
>>
>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>