linux-kernel - Re: [PATCH] /dev/zero: make private mapping full anonymous mapping

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Message-ID: <8e88bce5-3be3-4e84-a327-841c91906a79@lucifer.local>
Date: Tue, 14 Jan 2025 14:52:53 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Yang Shi <yang@...amperecomputing.com>, arnd@...db.de,
        gregkh@...uxfoundation.org, Liam.Howlett@...cle.com, vbabka@...e.cz,
        jannh@...gle.com, willy@...radead.org, liushixin2@...wei.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] /dev/zero: make private mapping full anonymous mapping

On Tue, Jan 14, 2025 at 02:01:32PM +0100, David Hildenbrand wrote:
> On 13.01.25 23:30, Yang Shi wrote:
> > When creating private mapping for /dev/zero, the driver makes it an
> > anonymous mapping by calling set_vma_anonymous().  But it just sets
> > vm_ops to NULL, vm_file is still valid and vm_pgoff is also file offset.
> >
> > This is a special case and the VMA doesn't look like either anonymous VMA
> > or file VMA.  It confused other kernel subsystem, for example, khugepaged [1].
> >
> > It seems pointless to keep such special case.  Making private /dev/zero>
> mapping a full anonymous mapping doesn't change the semantic of
> > /dev/zero either.
> >
> > The user visible effect is the mapping entry shown in /proc/<PID>/smaps
> > and /proc/<PID>/maps.
> >
> > Before the change:
> > ffffb7190000-ffffb7590000 rw-p 00001000 00:06 8                          /dev/zero
> >
> > After the change:
> > ffffb6130000-ffffb6530000 rw-p 00000000 00:00 0
> >
>
> Hm, not sure about this. It's actually quite consistent to have that output
> in smaps the way it is. You mapped a file at an offset, and it behaves like
> an anonymous mapping apart from that.
>
> Not sure if the buggy khugepaged thing is a good indicator to warrant this
> change.

Yeah, this is a user-facing fundamental change that hides information and
defies expectation so I mean - it's a no go really isn't it?

I'd rather we _not_ make this anon though, because isn't life confusing
enough David? I thought it was bad enough with 'anon, file and lol shmem'
but 'lol lol also /dev/zero' is enough to make me want to frolick in the
fields...

>
> --
> Cheers,
>
> David / dhildenb
>