linux-kernel - Re: [PATCH] [RFC] mm/fadvise: introduce POSIX_FADV

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Message-ID: <aSB25FFkLaJkbVfT@casper.infradead.org>
Date: Fri, 21 Nov 2025 14:27:48 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH] [RFC] mm/fadvise: introduce POSIX_FADV_MLOCK

On Fri, Nov 21, 2025 at 04:46:14AM +0000, Jaegeuk Kim wrote:
> On 11/21, Matthew Wilcox wrote:
> > On Fri, Nov 21, 2025 at 03:27:18AM +0000, Jaegeuk Kim wrote:
> > > This patch introduces a new POSIX_FADV_MLOCK which 1) invalidates the range of
> > > cached pages, 2) sets the mapping as inaccessible, 3) POSIX_FADV_WILLNEED loads
> > > pages directly to the inaccessible mapping.
> > 
> > ... what?
> > 
> > This seems like something which is completely different from mlock().
> > So it needs a different name.
> > 
> > But I don't understand the point of this, whatever it's called.  Need
> > more information.
> 
> So, the sequence that I'd like to optimize is mmap(MAP_POPULATE) followed
> by  mlock(). For example, mmap() takes 1 second to load 4GB data, and mlock()
> takes 330ms additionally in order to migrate all the pages into inaccessible
> map, IIUC.

Oh, so the MLOCK part is right, but the inaccessible() part is wrong.
Inaccessible is special weird guest_memfd crap that has all kinds of
side-effects that you don't want.

Wouldn't you get the same effect by calling mlock2(MLOCK_ONFAULT) and
then calling readahead() for the desired range?