Re: [PATCH v2 0/2] DAX bdev fixes - move flushing calls to FS

[Jan Kara]

On Wed 10-02-16 13:48:54, Ross Zwisler wrote:
> During testing of raw block devices + DAX I noticed that the struct
> block_device that we were using for DAX operations was incorrect.  For the
> fault handlers, etc. we can just get the correct bdev via get_block(),
> which is passed in as a function pointer, but for the *sync code and for
> sector zeroing we don't have access to get_block().  This is also an issue
> for XFS real-time devices, whenever we get those working.
> 
> Patch one of this series fixes the DAX sector zeroing code by explicitly
> passing in a valid struct block_device.
> 
> Patch two of this series fixes DAX *sync support by moving calls to
> dax_writeback_mapping_range() out of filemap_write_and_wait_range() and
> into the filesystem/block device ->writepages function so that it can
> supply us with a valid block device. This also fixes DAX code to properly
> flush caches in response to sync(2).
> 
> Thanks to Jan Kara for his initial draft of patch 2:
> https://lkml.org/lkml/2016/2/9/485
> 
> Here are the changes that I've made to that patch:
> 
> 1) For DAX mappings, only return after calling
> dax_writeback_mapping_range() if we encountered an error.  In the non-error
> case we still need to write back normal pages, else we lose metadata
> updates. 
> 
> 2) In dax_writeback_mapping_range(), move the new check for 
>         if (!mapping->nrexceptional || wbc->sync_mode != WB_SYNC_ALL)
> above the i_blkbits check.  In my testing I found cases where
> dax_writeback_mapping_range() was called for inodes with i_blkbits !=
> PAGE_SHIFT - I'm assuming these are internal metadata inodes?  They have no
> exceptional DAX entries to flush, so we have no work to do, but if we
> return error from the i_blkbits check we will fail the overall writeback
> operation.  Please let me know if it seems wrong for us to be seeing inodes
> set to use DAX but with i_blkbits != PAGE_SHIFT and I'll get more info.

So I'm wondering - how come S_DAX flag got set for inode where i_blkbis !=
PAGE_SHIFT? That would seem to be a bug? I specifically ordered the checks
like this to catch such issues.

> 3) In filemap_write_and_wait() and filemap_write_and_wait_range(), continue
> the writeback in the case that DAX is enabled but we only have a nonzero
> mapping->nrpages.  As with 1) and 2), I believe this is necessary to
> properly writeback metadata changes.  If this sounds wrong, please let me
> know and I'll get more info.

And I'm surprised here as well. If there are dax_mapping() inodes that have
pagecache pages, then we have issues with radix tree handling as well. So
how come dax_mapping() inodes have pages attached? If it is about block
device inodes, then I find it buggy, that S_DAX gets set for such inodes
when filesystem is mounted on them because in such cases we are IMO asking
for data corruption sooner rather than later...

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