Re: [PATCH mm-unstable v14 03/16] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

[Lorenzo Stoakes]

On Wed, Jan 28, 2026 at 09:38:37AM -0700, Nico Pache wrote:
> Hi Andrew,
>
> could you please apply the following fixup to avoid potentially using a stale
> VMA in the new writeback-retry logic for madvise collapse.
>
> Thank you!
> -- Nico
>
> ----8<----
> commit a9ac3b1bfa926dd707ac3a785583f8d7a0579578
> Author: Nico Pache <npache@xxxxxxxxxx>
> Date:   Fri Jan 23 16:32:42 2026 -0700
>
>     madvise writeback retry logic fix
>
>     Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 59e5a5588d85..2b054f7d9753 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2418,6 +2418,14 @@ static enum scan_result collapse_single_pmd(unsigned long
> addr,
>  	mmap_read_unlock(mm);
>  	*mmap_locked = false;
>  	result = collapse_scan_file(mm, addr, file, pgoff, cc);
> +
> +	if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
> +	    mapping_can_writeback(file->f_mapping)) {
> +		const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> +		const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> +
> +		filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> +	}
>  	fput(file);
>
>  	if (result != SCAN_PTE_MAPPED_HUGEPAGE)
> @@ -2840,19 +2848,8 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned
> long start,
>  			*lock_dropped = true;
>
>  		if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
> -			struct file *file = get_file(vma->vm_file);
> -			pgoff_t pgoff = linear_page_index(vma, addr);
> -
> -			if (mapping_can_writeback(file->f_mapping)) {
> -				loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> -				loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> -
> -				filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> -				triggered_wb = true;
> -				fput(file);
> -				goto retry;
> -			}
> -			fput(file);
> +			triggered_wb = true;
> +			goto retry;
>  		}

LGTM, with this in place you can add back my tag to the patch.

It'd be good to reference this in the commit message, but you can do that
on a respin.

So:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>