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

[David Hildenbrand]

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..

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

[1] 
https://lore.kernel.org/linux-mm/CAHk-=wi=XvCZ9r897LjEb4ZarLzLtKN1p+Fyig+F2fmQDF8GSA@xxxxxxxxxxxxxx/

So, nack from my side.

--
Cheers,

David / dhildenb