Messages in this thread | | | Date | Wed, 29 Apr 2015 10:06:53 +0300 | From | Boaz Harrosh <> | Subject | Re: Using pmem from a driver exposing a memory mapping (mmap) to userspace |
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On 04/28/2015 06:35 PM, Mathieu Desnoyers wrote: > Hi! > > I'm currently adaping lttng-modules to use DAX and pmem. > It will allow LTTng buffers to be recovered after a kernel > crash. I've moved pretty much all struct page pointers to > page frame numbers, as I remember being told that pmem does > not have struct page. > > Now I'm looking into adapting my mmap and page fault handler > implementation (based on struct page) to a page-frame number > based implementation when the ring buffer is backed by > persistent memory, which will probably not require any page > fault handler at all when based by pmem+dax memory.
There will be page-faults at lease once for every combination of application+page. Sure there may only be one per a+p until the application does a close on the file.
Your job can be simple if you use the pmem's inode. You know how each block-device is a mini file system with a single file. Use bdev->bd_inode to get to the one inode associated with your pmem bdev. Well this inode is IS_DAX(), so if you supply your own get_block() function to the DAX handlers you need not duplicate any mmap code at all.
(You can also use the same DAX infrastructure for the read/write_iter implementation)
> > My current work is in this branch: https://github.com/compudj/lttng-modules-dev/tree/persistent-memory-buffers > (see last commits) > > LTTng-modules supports both mmap() and splice(), but I plan > to only provide mmap() support for persistent memory, since > splice() really requires struct page. >
No splice just works fine. In-fact a NULL .splice_XXX vector will use the default_file_splice_read/write which does a copy and uses your regular read/write_iter vectors. So leave the .splice NULL and it will be supported by your read/write_iter interface.
> Are there existing driver mmap implementations doing similar > things, or do you have recommendations on how to implement > this ? >
DAX.c lib does all that you need. You only need your own translation from your device files to a chunk of pmem.
Its how I'd do it, good luck. CC me on the patches I'll review them.
Cheers Boaz
> Thanks, > Mathieu
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