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

[Lorenzo Stoakes (ARM)]

On Sat, Sep 05, 2026 at 05:17:57PM -0700, Andrew Morton wrote:
> On Fri, 4 Sep 2026 17:01:37 +0100 "Lorenzo Stoakes (ARM)" <ljs@xxxxxxxxxx> wrote:
>
> > Andrew - could you swap the patch out in-place with below? Thanks!
> >
>
> np.
>
> > 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>
>
> The failure mode looks pretty obscure, so I'm thinking that no backport
> is needed.

Well it's not even really a failure mode so much as a 'known limitation' :)
So yeah agreed.

--
Cheers, Lorenzo