Aneesh Kumar K.V wrote: > On Mon, May 11, 2009 at 11:08:56PM -0400, Theodore Tso wrote: >> On Thu, May 07, 2009 at 04:09:29PM +0530, Aneesh Kumar K.V wrote: >>> ext4_get_blocks_wrap does a block lookup requesting to >>> allocate new blocks. A lookup of blocks in prealloc area >>> result in setting the unwritten flag in buffer_head. So >>> a write to an unwritten extent will cause the buffer_head >>> to have unwritten and mapped flag set. Clear hte unwritten >>> buffer_head flag before requesting to allocate blocks. >>> >>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx> >> I've rewritten the commit changelog to this, which I believe more >> accurately describes the patch. Comments, please? >> >> ext4: Clear the unwritten buffer_head flag after the extent is initialized >> >> From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> >> >> The BH_Unwritten flag indicates that the buffer is allocated on disk >> but has not been written; that is, the disk was part of a persistent >> preallocation area. That flag should only be set when a get_blocks() >> function is looking up a inode's logical to physical block mapping. >> >> When ext4_get_blocks_wrap() is called with create=1, the uninitialized >> extent is converted into an initialized one, so the BH_Unwritten flag >> is no longer appropriate. Hence, we need to make sure the >> BH_Unwritten is not left set, to avoid the ensuing confusion and >> hilarty. >> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx> >> Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx> >> > > I think it is good. But one thing missing in the commit message is, > what happens if we do a write to prealloc space. Since a > get_block(create = 1) is now split into __get_block(create = 0 ) and > __get_block(create = 1). That would mean if we pass a buffer head with > BH_Unwritten cleared we will have > > > 1) buffer_head as BH_Unwritten cleared. > > 2) __get_block(create = 0 ) -> Since it is prealloc space we will have > BH_Unwritten set . Why do we need to set BH_Unwritten on a !create call at all? Or maybe another way of asking is, are there any !create callers of get_block who -want- BH_Unwritten set? Which is to say, should we just not be setting BH_Unwritten in get_block in the !create case, ever? The comment: /* + * The above get_blocks can cause the buffer to be + * marked unwritten. So clear the same. + */ + clear_buffer_unwritten(bh); is imho not helpful; to me it says "we -just- set this, so clear it!" It leaves me scratching my head. > 3) __get_block(create = 1) -> get the blocks out of prealloc space. > and retun with BH_Mapped set. > > That would imply we have BH_Unwritten and BH_Mapped set in the above > case which is wrong. So we need a BH_Unwritten clear between (2) and > (3). The patch does the same. May be we need to capture it in commit > message. Better in comments, I think. :) -Eric > -aneesh > > > -- 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
