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
