On Tue, Apr 19 2016, Jan Kara wrote:
> Currently DAX page fault locking is racy.
>
> CPU0 (write fault) CPU1 (read fault)
>
> __dax_fault() __dax_fault()
> get_block(inode, block, &bh, 0) -> not mapped
> get_block(inode, block, &bh, 0)
> -> not mapped
> if (!buffer_mapped(&bh))
> if (vmf->flags & FAULT_FLAG_WRITE)
> get_block(inode, block, &bh, 1) -> allocates blocks
> if (page) -> no
> if (!buffer_mapped(&bh))
> if (vmf->flags & FAULT_FLAG_WRITE) {
> } else {
> dax_load_hole();
> }
> dax_insert_mapping()
>
> And we are in a situation where we fail in dax_radix_entry() with -EIO.
>
> Another problem with the current DAX page fault locking is that there is
> no race-free way to clear dirty tag in the radix tree. We can always
> end up with clean radix tree and dirty data in CPU cache.
>
> We fix the first problem by introducing locking of exceptional radix
> tree entries in DAX mappings acting very similarly to page lock and thus
> synchronizing properly faults against the same mapping index. The same
> lock can later be used to avoid races when clearing radix tree dirty
> tag.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: NeilBrown <neilb@xxxxxxxx> (for the exception locking bits)
I really like how you have structured the code - makes it fairly
obviously correct!
Thanks,
NeilBrown
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