• Darrick J. Wong's avatar
    dm cache: fix writes to cache device in writethrough mode · b844fe69
    Darrick J. Wong authored
    The dm-cache writethrough strategy introduced by commit e2e74d61
    ("dm cache: fix race in writethrough implementation") issues a bio to
    the origin device, remaps and then issues the bio to the cache device.
    This more conservative in-series approach was selected to favor
    correctness over performance (of the previous parallel writethrough).
    However, this in-series implementation that reuses the same bio to write
    both the origin and cache device didn't take into account that the block
    layer's req_bio_endio() modifies a completing bio's bi_sector and
    bi_size.  So the new writethrough strategy needs to preserve these bio
    fields, and restore them before submission to the cache device,
    otherwise nothing gets written to the cache (because bi_size is 0).
    
    This patch adds a struct dm_bio_details field to struct per_bio_data,
    and uses dm_bio_record() and dm_bio_restore() to ensure the bio is
    restored before reissuing to the cache device.  Adding such a large
    structure to the per_bio_data is not ideal but we can improve this
    later, for now correctness is the important thing.
    
    This problem initially went unnoticed because the dm-cache test-suite
    uses a linear DM device for the dm-cache device's origin device.
    Writethrough worked as expected because DM submits a *clone* of the
    original bio, so the original bio which was reused for the cache was
    never touched.
    Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
    Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
    Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
    Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
    b844fe69
dm-cache-target.c 63.4 KB