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

[Andrew Morton]

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.