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

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Message-ID: <6b87e597-e3e6-4e46-a4c2-ca358d52e557@os.amperecomputing.com>
Date: Tue, 14 Jan 2025 09:01:06 -0800
From: Yang Shi <yang@...amperecomputing.com>
To: David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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 1/14/25 7:06 AM, David Hildenbrand wrote:
> 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.

I admit this may be a concern, but I doubt who really care about it...

>>
>> 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* :)

The behavior won't be changed.