Re: [BUG/RFC] mm/madvise: MADV_WILLNEED skips swapped file-backed COW pages

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Aug 31, 2026 at 11:24:48AM -0700, Mike Kaplinskiy wrote:
> Hi,
>
> I'm seeing a strange behavior when using MADV_WILLNEED to schedule
> swap page-in. It seems MADV_WILLNEED does not schedule swap reads for
> swapped-out COW pages in an ordinary file-backed MAP_PRIVATE mapping.
>
> I reproduced this on Linux 7.0.14 on aarch64, but I think this code
> hasn't changed in a while. mm/madvise.c:madvise_willneed walks swap

It's more a known limitation of MADV_WILLNEED that has existed forever.

The issue is that every single MAP_PRIVATE-file backed mapping would then
need to be walked twice, once via page tables -> swap cache and once via
readahead.

But you could figure out if it was CoW'd...

Something like:

diff --git a/mm/madvise.c b/mm/madvise.c
index bc6a7dc73021..33ff3ba69c7f 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -297,10 +297,12 @@ static long madvise_willneed(struct madvise_behavior *madv_behavior)
        loff_t offset;

 #ifdef CONFIG_SWAP
-       if (!file) {
+       if (!file || (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;
        }

        if (shmem_mapping(file->f_mapping)) {

(This also happens to fix a bug there with MAP_PRIVATE-/dev/zero though
that'll get fixed with my upcoming series anyway :)

The vma->anon_vma check ensures CoW'd pages have actually been mapped in.

But then you'd have to do two walks for every single CoW'd MAP_PRIVATE-file
backed mapping.

The majority of the anon walk would be a no-op also.

> PTEs only when vma->vm_file is NULL, and sends other file-backed
> mappings to vfs_fadvise(POSIX_FADV_WILLNEED). This skips the case of
> MAP_PRIVATE mappings with changes, which frequently happens for
> libraries/binaries with relocations and/or writable globals.
>
> The code is at the top of
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/madvise.c#n282

That code is skipping non-swap entries for a swapped out shmem folio? I
think that's irrelevant to this.

> .
>
> Minimal reproducer attached. Would there be interest in changing this
> path to prefetch private copies in addition to the readahead?

I mean I'd like to hear from others, if OK I can send the patch above.

Are we concerned about the inefficiency of this?

Or dropping the mmap lock right after and faulting in the file-backed bits?

I guess if you're doing an MADV_WILLNEED you are fine with it taking a bit
of extra time to swap stuff in.

>
> Thanks,
> Mike

--
Cheers, Lorenzo