Re: [PATCH 1/1] mm/mmap: allow MAP_DROPPABLE | MAP_PRIVATE in mmap()

[Lorenzo Stoakes]

Agree with David, NACK.

On Mon, Jan 20, 2025 at 08:45:07AM +0100, David Hildenbrand wrote:
> On 20.01.25 02:26, Lance Yang wrote:
> > Currently, mmap() fails with `-EINVAL` when both MAP_DROPPABLE and
> > MAP_PRIVATE are specified. This behavior might be inconsistent, as the
> > implementation of MAP_DROPPABLE under the hood already includes the
> > semantics of MAP_PRIVATE. So, IMO, whether MAP_PRIVATE is explicitly
> > specified or not, it should work as expected.
> >
> > For example, when mmap() is called with `MAP_DROPPABLE | MAP_ANONYMOUS`,
> > it creates a private anonymous mapping. Users can verify this behavior
> > via `/proc/self/smaps`, where the resulting VMA is marked with the `dp`
> > (MAP_DROPPABLE) flag, and the `Private_*` fields confirm private memory
> > semantics. The output for a 2MiB mapping with these flags might look like:
>
> Note that "Private_" in the stats has *nothing* to do with MAP_PRIVATE.
>
> >
> > ```
> > f433ace00000-f433ad000000 rw-p 00000000 00:00 0
> > Size:               2048 kB
> > KernelPageSize:        4 kB
> > MMUPageSize:           4 kB
> > Rss:                2048 kB
> > Pss:                2048 kB
> > Pss_Dirty:          2048 kB
> > Shared_Clean:          0 kB
> > Shared_Dirty:          0 kB
> > Private_Clean:         0 kB
> > Private_Dirty:      2048 kB
> > Referenced:         2048 kB
> > Anonymous:          2048 kB
> > ...
> > VmFlags: rd wr mr mw me nr wf dd dp
> > ```
> >
> > This patch changes mmap() to allow the combination of `MAP_DROPPABLE |
> > MAP_PRIVATE`. For mmap(), at least one of MAP_PRIVATE or MAP_SHARED could
> > be explicitly specified, regardless of the combination with other `MAP_*`
> > flags.
> >
> > Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
>
> "How about we just say that VM_DROPPABLE really is something separate
> from MAP_PRIVATE or MAP_SHARED..

Which is also how I view it. I -really- do not want to add a weird situation too
where people wonder whether _not_ setting MAP_PRIVATE infers some different
semantics.

This mode is aggregate in behaviour by design and intended to be _specifically_
asked for, not in conjection with other map flags.

>
> And then we make the rule be that VM_DROPPABLE is never dumped and
> always dropped on fork, just to make things simpler." [1]

Yup.

>
> [1] https://lore.kernel.org/linux-mm/CAHk-=wi=XvCZ9r897LjEb4ZarLzLtKN1p+Fyig+F2fmQDF8GSA@xxxxxxxxxxxxxx/
>
> So, nack from my side.

Also, mine.

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