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

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Message-ID: <0e616478-96d4-41e9-b6c1-fa641c36fba3@gmail.com>
Date: Fri, 7 Nov 2025 10:12:02 +0100
From: "David Hildenbrand (Red Hat)" <davidhildenbrandkernel@...il.com>
To: "Garg, Shivank" <shivankg@....com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
 Lance Yang <lance.yang@...ux.dev>, Vlastimil Babka <vbabka@...e.cz>,
 Jann Horn <jannh@...gle.com>, zokeefe@...gle.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: madvise(MADV_COLLAPSE) fails with EINVAL on dirty file-backed
 text pages

> 
> 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@...hat.com>
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.