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

[Yang Shi]

On 1/14/25 9:02 AM, David Hildenbrand wrote:
> 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.

For private mapping. Shared mapping is equivalent to shmem.

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

Interesting... Didn't know this before.

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

Let's make /dev/zero as real as anonymous memory :)

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