Re: [PATCH] mm/madvise: swap in CoW'd MAP_PRIVATE-file mappings on MADV_WILLNEED

[Pedro Falcato]

On Fri, Sep 04, 2026 at 04:12:53PM +0100, Lorenzo Stoakes (ARM) wrote:
> Currently MADV_WILLNEED treats file-backed and pure anonymous mappings
> entirely separately - using POSIX_FADV_WILLNEED (equivalent of a readahead)
> for the former and a tree walk and swap in to swap cache for the latter.
> 
> MAP_PRIVATE-file backed mappings straddle the two and currently get treated
> as if they were purely file-backed, meaning any swapped out private pages
> remain swapped out.
> 
> Resolve the issue by explicitly checking for CoW'd MAP_PRIVATE-file backed
> mappings and performing both walks in this case.
> 
> Since the logic checks for vma->anon_vma this means un-CoW'd
> MAP_PRIVATE-file backed mappings retain only the single file walk.
> 
> Reported-by: Mike Kaplinskiy <mike@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/CABeknB_S2XJSHFgnHdgnN0rjzHhH4oQJs_APq9fvxHztQ_pgiA@xxxxxxxxxxxxxx/
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---
>  mm/madvise.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 73c2901b9adb..d0510dd49dde 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -297,10 +297,12 @@ static long madvise_willneed(struct madvise_behavior *madv_behavior)
>  	loff_t offset;
>  
>  #ifdef CONFIG_SWAP
> -	if (!file) {
> +	if (!file || (vma_is_cow_mapping(vma) && vma->anon_vma)) {

Couldn't this all be simplified to

	if (vma->anon_vma) {

? swapin needs anon pages to have been faulted-in. Non-cow mappings won't
have an anon_vma, nor will fully empty anonymous VMAs (and that's fine).
Right?

Regardless, looks cromulent

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

-- 
Pedro