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

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Message-ID: <75dbebfa-d13c-436e-a9ac-87f98b140cc6@redhat.com>
Date: Tue, 14 Jan 2025 16:06:21 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 14.01.25 15:52, Lorenzo Stoakes wrote:
> 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...

I recall there are users that rely on this memory to get the shared 
zeropage on reads etc (in comparison to shmem!), so I better not ... 
mess with this *at all* :)

-- 
Cheers,

David / dhildenb