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

[Lorenzo Stoakes (ARM)]

On Mon, Sep 07, 2026 at 01:51:20PM +0200, David Hildenbrand (Arm) wrote:
> > 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)

Ack, though it's kinda obviously functionally equivalent (or so I felt :P).

I had a local LLM check it FWIW.

>
> >
> > 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>

Thanks!

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo