Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax, dma-to-storage, and swap

[Dan Williams]

On Mon, Aug 14, 2017 at 5:40 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Sun 13-08-17 13:31:45, Dan Williams wrote:
>> On Sun, Aug 13, 2017 at 2:24 AM, Christoph Hellwig <hch@xxxxxx> wrote:
>> > Thay being said I think we absolutely should support RDMA memory
>> > registrations for DAX mappings.  I'm just not sure how S_IOMAP_IMMUTABLE
>> > helps with that.  We'll want a MAP_SYNC | MAP_POPULATE to make sure
>> > all the blocks are polulated and all ptes are set up.  Second we need
>> > to make sure get_user_page works, which for now means we'll need a
>> > struct page mapping for the region (which will be really annoying
>> > for PCIe mappings, like the upcoming NVMe persistent memory region),
>> > and we need to gurantee that the extent mapping won't change while
>> > the get_user_pages holds the pages inside it.  I think that is true
>> > due to side effects even with the current DAX code, but we'll need to
>> > make it explicit.  And maybe that's where we need to converge -
>> > "sealing" the extent map makes sense as such a temporary measure
>> > that is not persisted on disk, which automatically gets released
>> > when the holding process exits, because we sort of already do this
>> > implicitly.  It might also make sense to have explicitl breakable
>> > seals similar to what I do for the pNFS blocks kernel server, as
>> > any userspace RDMA file server would also need those semantics.
>>
>> Ok, how about a MAP_DIRECT flag that arranges for faults to that range to:
>>
>>     1/ only succeed if the fault can be satisfied without page cache
>>
>>     2/ only install a pte for the fault if it can do so without
>> triggering block map updates
>>
>> So, I think it would still end up setting an inode flag to make
>> xfs_bmapi_write() fail while any process has a MAP_DIRECT mapping
>> active. However, it would not record that state in the on-disk
>> metadata and it would automatically clear at munmap time. That should
>> be enough to support the host-persistent-memory, and
>> NVMe-persistent-memory use cases (provided we have struct page for
>> NVMe). Although, we need more safety infrastructure in the NVMe case
>> where we would need to software manage I/O coherence.
>
> Hum, this proposal (and the problems you are trying to deal with) seem very
> similar to Peter Zijlstra's mpin() proposal from 2014 [1], just moved to
> the DAX area (and so additionally complicated by the fact that filesystems
> now have to care). The patch set was not merged due to lack of interest I
> think but it looked sensible and the proposed API would make sense for more
> stuff than just DAX so maybe it would be better than MAP_DIRECT flag?

Interesting, but I'm not sure I see the correlation. mm_mpin() makes a
"no-fault" guarantee and fixes the accounting of locked System RAM.
MAP_DIRECT still allows faults, and DAX mappings don't consume System
RAM so the accounting problem is not there for DAX. mm_pin() also does
not appear to have a relationship to a file backed memory like mmap
allows.