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

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Message-ID: <aSDaXN-JhhfljCeT@google.com>
Date: Fri, 21 Nov 2025 21:32:12 +0000
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: Matthew Wilcox <willy@...radead.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 11/21, Matthew Wilcox wrote:
> On Fri, Nov 21, 2025 at 07:52:02PM +0000, Jaegeuk Kim wrote:
> > On 11/21, Jaegeuk Kim wrote:
> > > On 11/21, Matthew Wilcox wrote:
> > > > 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?
> > > 
> > > Oh, thank you. Let me try.
> > 
> > After checking the code and experiment, I don't think that gives what we need.
> > That flag skips populate_vma_page_range only, but we need to allocate pages
> > in the inaccessible mapping and fill the pages afterwards.
> 
> Then either I don't understand what you're trying to do, or you don't
> understand what the inaccessible mapping is for.  Is this just for
> speeding up mlock() as you suggested earlier, or are you genuinely
> trying to do something with the inaccessible mapping?

The latter. I'd like to propose a new read flow with the inaccessible mapping.

As-Is:
 mmap() -> fadvise(fd, POSIX_FADV_WILLNEED) -> mlock()

1. fadvise() proposal
 mmap() -> fadvise(fd, POSIX_FADV_MLOCK)
 : all the pages will be loaded into inaccessible page cache directly

2. mlock2() proposal
 mmap() -> mlock2(MLOCK_ONFAULT) -> madvise(MADV_POPULATE_READ)

If you mean #2, I need to find whether we can get the space for madvise, since
we have only fd when reading the pages. And, also I need to find a way to handle
the folio order directly instead of starging from 0 in madvise() path.
Let me think about it.