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

[Vlastimil Babka (SUSE)]

On 9/7/26 14:32, Lorenzo Stoakes (ARM) wrote:
> 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.

Yeah, as we know madvise() just an advice after all, the kernel is free to
ignore it :)

(MADV_DONTNEED and friends enter the chat)

> --
> Cheers, Lorenzo