On 14.01.25 18:38, Yang Shi wrote:
On 1/14/25 9:23 AM, David Hildenbrand wrote:
On 14.01.25 18:01, Yang Shi wrote:
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...
There is an example in the man page [1] about /proc/self/map_files/.
I assume that will also change here.
IIUC, that example is specific to "anonymous shared memory" created by
shared mapping of /dev/zero.
Note that MAP_PRIVATE of /dev/zero will also make it appear in the same
way right now (I just tried).
The example is about MAP_FILE in general, not just MAP_SHARED IIUC.
--
Cheers,
David / dhildenb
