On Thu, Jan 28, 2016 at 01:38:58PM -0800, Christoph Hellwig wrote: > On Thu, Jan 28, 2016 at 12:35:04PM -0700, Ross Zwisler wrote: > > There are a number of places in dax.c that look up the struct block_device > > associated with an inode. Previously this was done by just using > > inode->i_sb->s_bdev. This is correct for inodes that exist within the > > filesystems supported by DAX (ext2, ext4 & XFS), but when running DAX > > against raw block devices this value is NULL. This causes NULL pointer > > dereferences when these block_device pointers are used. > > It's also wrong for an XFS file system with a RT device.. > > > +#define DAX_BDEV(inode) (S_ISBLK(inode->i_mode) ? I_BDEV(inode) \ > > + : inode->i_sb->s_bdev) > > .. but this isn't going to fix it. You must use a bdev returned by > get_blocks or a similar file system method. Jan & Dave, Before I start in on a solution to this issue I just wanted to confirm that DAX can rely on the fact that the filesystem's get_block() call will reliably set bh->b_bdev for non-error returns. From this conversation between Jan & Dave: https://lkml.org/lkml/2016/1/7/723 " > No. The real problem is a long-standing abuse of struct buffer_head to be > used for passing block mapping information (it's on my todo list to remove > that at least from DAX code and use cleaner block mapping interface but > first I want basic DAX functionality to settle down to avoid unnecessary > conflicts). Filesystem is not supposed to touch bh->b_bdev. That has not been true for a long, long time. e.g. XFS always rewrites bh->b_bdev in get_blocks because the file may not reside on the primary block device of the filesystem. i.e.: /* * If this is a realtime file, data may be on a different device. * to that pointed to from the buffer_head b_bdev currently. */ bh_result->b_bdev = xfs_find_bdev_for_inode(inode); > If you need > that filled in, set it yourself in before passing bh to the block mapping > function. That may be true, but we cannot assume that the bdev coming back out of get_block is the same one that was passed in. " It sounds like this is always true for XFS, and from looking at the ext4 code I think this is true there as well because bh->b_bdev is set in ext4_dax_mmap_get_block() via map_bh(). Relying on the bh->b_bdev returned by get_block() is correct, yea? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
