linux-kernel - Re: [PATCH 1/2] mm/khugepaged: do synchronous writeback for MADV

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Message-ID: <aRH0ANHpP8VN1j9r@casper.infradead.org>
Date: Mon, 10 Nov 2025 14:17:36 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Shivank Garg <shivankg@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
	Lance Yang <lance.yang@...ux.dev>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Zach O'Keefe <zokeefe@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	Branden Moore <Branden.Moore@....com>
Subject: Re: [PATCH 1/2] mm/khugepaged: do synchronous writeback for
 MADV_COLLAPSE

On Mon, Nov 10, 2025 at 01:52:46PM +0000, Lorenzo Stoakes wrote:
> On Mon, Nov 10, 2025 at 01:47:10PM +0000, Matthew Wilcox wrote:
> > On Mon, Nov 10, 2025 at 11:32:53AM +0000, Shivank Garg wrote:
> > > When MADV_COLLAPSE is called on file-backed mappings (e.g., executable
> > > text sections), the pages may still be dirty from recent writes. The
> >
> > That explanation derails my brain entirely.  Text isn't writable!  How
> > can the pages be dirty and file-backed text?
> 
> Because they just compiled it and it's not been written back to disk yet :)

So we can avoid these patches simply by changing userspace to call
fsync()?  Cool!