Re: [PATCH V4 07/13] fs: Add locking for a dynamic address space operations state

[Jan Kara]

On Tue 25-02-20 11:09:37, Dave Chinner wrote:
> /me wonders if the best thing to do is to add a ->fault callout to
> tell the filesystem to lock/unlock the inode right up at the top of
> the page fault path, outside even the mmap_sem.  That means all the
> methods that the page fault calls are protected against S_DAX
> changes, and it gives us a low cost method of serialising page
> faults against DIO (e.g. via inode_dio_wait())....

Well, that's going to be pretty hard. The main problem is: you cannot
lookup VMA until you hold mmap_sem and the inode is inside the VMA. And
this is a fundamental problem because until you hold mmap_sem, the address
space can change and thus the virtual address you are faulting can be
changing inode it is mapped to. So you would have to do some dance like:

lock mmap_sem
lookup vma
get inode reference
drop mmap_sem
tell fs about page fault
lock mmap_sem
is the vma still the same?

And I'm pretty confident the overhead will be visible in page fault
intensive workloads...

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR