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.. 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
