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

[David Hildenbrand (Arm)]

> And it's a real improvement to have unfaulted anon skip...
> 
> But I find that version is documenting what's going on a lot less.
> 
> Since both anon and MAP_PRIVATE file-backed are CoW mappings we could just
> reference that.
> 
> Andrew - could you swap the patch out in-place with below? Thanks!

A proper v2 would have been nice ;)

(and would trigger sashiko and friends properly)

> 
> Cheers, Lorenzo
> 
> ----8<----
> From a706374a1186b6b4409a0e760e8728bcadba0c96 Mon Sep 17 00:00:00 2001
> From: "Lorenzo Stoakes (ARM)" <ljs@xxxxxxxxxx>
> Date: Thu, 3 Sep 2026 20:08:39 +0100
> Subject: [PATCH] mm/madvise: swap in CoW'd MAP_PRIVATE-file mappings on
>  MADV_WILLNEED
> 
> 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 | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 73c2901b9adb..963337f93a7a 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -297,11 +297,12 @@ static long madvise_willneed(struct madvise_behavior *madv_behavior)
>  	loff_t offset;
> 
>  #ifdef CONFIG_SWAP
> -	if (!file) {
> +	if (vma_is_cow_mapping(vma) && vma->anon_vma) {
>  		walk_page_range_vma(vma, start, end, &swapin_walk_ops, vma);
>  		lru_add_drain(); /* Push any new pages onto the LRU now */
> -		return 0;
>  	}
> +	if (!file)
> +		return 0;
> 
LGTM

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David