On Wed 13-01-16 00:08:29, Ross Zwisler wrote: > On Tue, Jan 12, 2016 at 10:34:58AM +0100, Jan Kara wrote: > > On Thu 07-01-16 22:27:51, Ross Zwisler wrote: > > > In __dax_pmd_fault() we currently assume that get_block() will always set > > > bh.b_bdev and we unconditionally dereference it in __dax_dbg(). This > > > assumption isn't always true - when called for reads of holes > > > ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is never > > > set. I hit this BUG while testing the DAX PMD fault path. > > > > > > Instead, initialize bh.b_bdev before passing bh into get_block(). It is > > > possible that the filesystem's get_block() will update bh.b_bdev, and this > > > is fine - we just want to initialize bh.b_bdev to something reasonable so > > > that the calls to __dax_dbg() work and print something useful. > > > > > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx> > > > > Looks good. But don't you need to do the same for __dax_fault(), > > dax_zero_page_range() and similar places passing bh to dax functions? > > > > Honza > > I don't think we need it anywhere else. The only reason that we need to > initialize the bh.b_bdev manually in the __dax_pmd_fault() path is that if the > get_block() call ends up finding a hole (so doesn't fill out b_bdev) we still > go through the dax_pmd_dbg() path to print an error message, which uses > b_bdev. I believe that in the other paths where we hit a hole, such as > __dax_fault(), we don't use b_bdev because we don't have the same error path > prints, and the regular code for handling holes doesn't use b_bdev. > > That being said, if you feel like it's cleaner to initialize it > everywhere so everything is consistent and we don't have to worry about > it, I'm fine to make the change. Well, it seems more futureproof to me. In case someone decides to add some debug message later on... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>