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

[David Hildenbrand <david@redhat.com>]

On 14.01.25 16:06, 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.
>>
>> 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* :)

Heh, and I recall reading something about odd behavior of /dev/zero and 
some interesting history [1].

"
Unlike /dev/null, /dev/zero may be used as a source, not only as a sink 
for data. All write operations to /dev/zero succeed with no other 
effects. However, /dev/null is more commonly used for this purpose.

When /dev/zero is memory-mapped, e.g., with mmap, to the virtual address 
space, it is equivalent to using anonymous memory; i.e. memory not 
connected to any file.
"

"equivalent to using anonymous memory" is interesting.


Also, /dev/zero was there before MAP_ANONYMOUS was invented according to 
[1], which is quite interesting.

... so this is anonymous memory as "real" as it can get :)


[1] https://en.wikipedia.org/wiki//dev/zero

-- 
Cheers,

David / dhildenb