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

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

Hi Bernd,


On 5/2/23 15:37, Bernd Schubert wrote:
> Hi Hao,
>
> On 5/2/23 09:28, Hao Xu wrote:
>> 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.
>
> Yeah it should work, but I think what you want is "DAX" - can you try 
> to enable it?
>
> fuse_i.h:    FUSE_DAX_ALWAYS,    /* "-o dax=always" */
> fuse_i.h:    FUSE_DAX_NEVER,        /* "-o dax=never" */
> fuse_i.h:    FUSE_DAX_INODE_USER,    /* "-o dax=inode" */
>
>
>
> Hope it helps,
> Bernd


Thanks for your suggestion, the thing is most IO in our case are small 
random read to small files. I believe Dax should help but doubt it may 
be not too much.

Another reason we are not leveraging DAX is the hypervisor software we 
use doesn't support DAX well. So I think the feature in this RFC should 
be good for

some non-DAX use cases (like ours)


Thanks,

Hao