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SubjectRe: [PATCH 2/2] dax: fix data corruption due to stale mmap reads
On Tue, Apr 25, 2017 at 01:10:43PM +0200, Jan Kara wrote:
> Hum, but now thinking more about it I have hard time figuring out why write
> vs fault cannot actually still race:
> CPU1 - write(2) CPU2 - read fault
> dax_iomap_pte_fault()
> ->iomap_begin() - sees hole
> dax_iomap_rw()
> iomap_apply()
> ->iomap_begin - allocates blocks
> dax_iomap_actor()
> invalidate_inode_pages2_range()
> - there's nothing to invalidate
> grab_mapping_entry()
> - we add zero page in the radix
> tree & map it to page tables
> Similarly read vs write fault may end up racing in a wrong way and try to
> replace already existing exceptional entry with a hole page?

Yep, this race seems real to me, too. This seems very much like the issues
that exist when a thread is doing direct I/O. One thread is doing I/O to an
intermediate buffer (page cache for direct I/O case, zero page for us), and
the other is going around it directly to media, and they can get out of sync.

IIRC the direct I/O code looked something like:

1/ invalidate existing mappings
2/ do direct I/O to media
3/ invalidate mappings again, just in case. Should be cheap if there weren't
any conflicting faults. This makes sure any new allocations we made are
faulted in.

I guess one option would be to replicate that logic in the DAX I/O path, or we
could try and enhance our locking so page faults can't race with I/O since
both can allocate blocks.

I'm not sure, but will think on it.

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