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
