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... > > -- > Cheers, > > David / dhildenb >
