Re: madvise(MADV_COLLAPSE) fails with EINVAL on dirty file-backed text pages

[Lance Yang]

On 2025/11/7 17:12, David Hildenbrand (Red Hat) wrote:
> > 5. Yes, I'm calling madvise(MADV_COLLAPSE) on the text portion of the 
> > executable, using the address
> >     range obtained from /proc/self/maps. IIUC, this should benefit 
> > applications by reducing ITLB pressure.
> >
> > I agree with the suggestions to either Return EAGAIN instead of EINVAL 
> > or At minimum, document the
> > EINVAL return for dirty pages. I'm happy to work on a patch.
>
> Of course, we could detect that we are in MADV_COLLAPSE and simply 
> writeback ourselves. After all,
> user space asked for a collapse, and it's not khugepaged that will 
> simple revisit it later.
>
> I did something similar in
>
> commit ab73b29efd36f8916c6cc9954e912c4723c9a1b0
> Author: David Hildenbrand <david@xxxxxxxxxx>
> Date:   Fri May 16 14:39:46 2025 +0200
>
>      s390/uv: Improve splitting of large folios that cannot be split 
> while dirty
>      Currently, starting a PV VM on an iomap-based filesystem with large
>      folio support, such as XFS, will not work. We'll be stuck in
>      unpack_one()->gmap_make_secure(), because we can't seem to make 
> progress
>      splitting the large folio.
>
> Where I effectively use filemap_write_and_wait_range().
>
> It could be used early to writeback the whole range to collapse once, 
> possibly.

Exactly!

Since MADV_COLLAPSE is a best-effort thing, having the kernel use
something like filemap_write_and_wait_range() to writeback the pages
before collapsing is likely what users would expect.

Anyway, they just want to get a THP, whether the pages are dirty or
clean :)