Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[Baolin Wang]

On 3/27/26 9:42 AM, Zi Yan wrote:
collapse_file() requires FSes supporting large folio with at least
PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem with
huge option turned on also sets large folio order on mapping, so the check
also applies to shmem.

While at it, replace VM_BUG_ON with returning failure values.

Signed-off-by: Zi Yan <z...@nvidia.com>
---
  mm/khugepaged.c | 7 +++++--
  1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d06d84219e1b..45b12ffb1550 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct mm_struct 
*mm, unsigned long addr,
        int nr_none = 0;
        bool is_shmem = shmem_file(file);
  
-	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
-       VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
+       /* "huge" shmem sets mapping folio order and passes the check below */
+       if (mapping_max_folio_order(mapping) < PMD_ORDER)
+               return SCAN_FAIL;

This is not true for anonymous shmem, since its large order allocation 
logic is similar to anonymous memory. That means it will not call 
mapping_set_large_folios() for anonymous shmem.

So I think the check should be:

if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
     return SCAN_FAIL;