On 14.01.25 18:20, Yang Shi wrote:
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.
"shared anonymous memory", yes.
--
Cheers,
David / dhildenb
