On Tue 14-07-15 09:48:51, Matthew Wilcox wrote: > On Tue, Jul 14, 2015 at 11:02:46AM +0200, Jan Kara wrote: > > On Mon 13-07-15 11:26:15, Matthew Wilcox wrote: > > > On Mon, Jul 13, 2015 at 05:16:10PM +0200, Jan Kara wrote: > > > > On Fri 03-07-15 11:15:11, Matthew Wilcox wrote: > > > > > From: Matthew Wilcox <willy@xxxxxxxxxxxxxxx> > > > > > > > > > > Currently, if ext4's get_block encounters a hole, it does not modify the > > > > > buffer_head. That's fine for many callers, but for DAX, it's useful to > > > > > know how large the hole is. XFS already returns the length of the hole, > > > > > so this improvement should not confuse any callers. > > > > > > > > > > Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx> > > > > > > > > So I'm somewhat wondering: What is the reason of BH_Uptodate flag being > > > > set? I can see the XFS sets it in some cases as well but the use of the > > > > flag isn't really clear to me... > > > > > > No clue. I'm just following the documentation in buffer.c: > > > > > > * NOTE! All mapped/uptodate combinations are valid: > > > * > > > * Mapped Uptodate Meaning > > > * > > > * No No "unknown" - must do get_block() > > > * No Yes "hole" - zero-filled > > > * Yes No "allocated" - allocated on disk, not read in > > > * Yes Yes "valid" - allocated and up-to-date in memory. > > > > OK, but that speaks about buffer head attached to a page. get_block() > > callback gets a temporary bh (at least in some cases) only so that it can > > communicate result of block mapping. And BH_Uptodate should be set only if > > data in the buffer is properly filled (which cannot be the case for > > temporary bh which doesn't have *any* data) and it simply isn't the case > > even for bh attached to a page because ext4 get_block() functions don't > > touch bh->b_data at all. So I just wouldn't set BH_Uptodate in get_block() > > at all.. > > OK, but how should DAX then distinguish between an old-style filesystem > (like current ext4) which reports "unknown" and leaves b_size untouched > when it encounters a hole, versus a new-style filesystem (XFS, ext4 with > this patch) which wants to report the size of a hole in b_size? The use > of Uptodate currently distinguishes the two cases. > > Plus, why would you want bh's to be treated differently, depending on > whether they're stack-based or attached to a page? That seems even more > confusing than bh's already are. Well, you may want to treat them differently because they *are* different. For example touching b_size of page-attached buffer_head is a no-go. get_block() interface is abusing buffer_head structure for historical reasons. Seeing you have hit issues with using buffer_head for passing mapping information I agree with Dave that we should convert DAX code to use iomaps instead of cluttering get_block() via buffer_head further. You can lift struct iomap from include/linux/exportfs.h (and related constant definitions) and use it for passing map information. It should be quite straightforward and simple now that DAX doesn't have many users. We will have: typedef int (iomap_fn_t)(struct inode *inode, loff_t offset, u64 length, bool create, struct iomap *iomap); and DAX functions will take this instead of get_block_t. Adding a wrapper to ext4_map_blocks() to work as iomap_fn_t is pretty straightforward as well. I'm sorry we didn't come up with this immediately when you started implementing DAX... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
