[RFC] FUSE: add another flag to support shared mmap in FOPEN_DIRECT_IO mode

[Hao Xu <hao.xu@linux.dev>]

Hi all,

 From discussion with Bernd, I get that FOPEN_DIRECT_IO is designed for 
those user cases where users want strong coherency like network 
filesystems, where one server serves multiple remote clients. And thus 
shared mmap is disabled since local page cache existence breaks this 
kind of coherency.

But here our use case is one virtiofs daemon serve one guest vm, We use 
FOPEN_DIRECT_IO to reduce memory footprint not for coherency. So we 
expect shared mmap works in this case. Here I suggest/am implementing 
adding another flag to indicate this kind of cases----use 
FOPEN_DIRECT_IO not for coherency----so that shared mmap works.


Thanks,

Hao