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. -- 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
