-----Original Message----- From: Jan Kara [mailto:jack@xxxxxxx] Sent: Thursday, January 15, 2015 4:41 AM To: ross.zwisler@xxxxxxxxxxxxxxx Cc: akpm@xxxxxxxxxxxxxxxxxxxx; Dilger, Andreas; axboe@xxxxxxxxx; boaz@xxxxxxxxxxxxx; david@xxxxxxxxxxxxx; hch@xxxxxx; jack@xxxxxxx; kirill.shutemov@xxxxxxxxxxxxxxx; mathieu.desnoyers@xxxxxxxxxxxx; Wilcox, Matthew R; rdunlap@xxxxxxxxxxxxx; tytso@xxxxxxx; mm-commits@xxxxxxxxxxxxxxx; linux-ext4@xxxxxxxxxxxxxxx Subject: Re: + ext4-add-dax-functionality.patch added to -mm tree On Mon 12-01-15 15:11:17, Andrew Morton wrote: > +#ifdef CONFIG_FS_DAX > +static int ext4_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf) > +{ > + return dax_fault(vma, vmf, ext4_get_block); > + /* Is this the right get_block? */ You can remove the comment. It is the right get_block function. Are you sure it shouldn't be ext4_get_block_write, or _write_nolock? According to the comments, ext4_get_block() doesn't allocate uninitialized extents, which we do want it to do. > diff -puN fs/ext4/inode.c~ext4-add-dax-functionality fs/ext4/inode.c > --- a/fs/ext4/inode.c~ext4-add-dax-functionality > +++ a/fs/ext4/inode.c > @@ -657,6 +657,18 @@ has_zeroout: > return retval; > } > > +static void ext4_end_io_unwritten(struct buffer_head *bh, int uptodate) > +{ > + struct inode *inode = bh->b_assoc_map->host; > + /* XXX: breaks on 32-bit > 16GB. Is that even supported? */ That should be 16 TB if I'm doing the math right - 32-bit block number * block size (4k) = 16 TB. And that's the max limit of ext4 (as logical file offset in blocks has to fit in 32-bits for ext4). So I think you can just remove the comment. But also see comment below. Blargh, yes, you're right. > @@ -694,6 +706,11 @@ static int _ext4_get_block(struct inode > > map_bh(bh, inode->i_sb, map.m_pblk); > bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags; > + if (IS_DAX(inode) && buffer_unwritten(bh) && !io_end) { > + bh->b_assoc_map = inode->i_mapping; > + bh->b_private = (void *)(unsigned long)iblock; > + bh->b_end_io = ext4_end_io_unwritten; > + } So why is this needed? It would deserve a comment. It confuses me in particular because: 1) This is a often a phony bh used just as a container for passed data and b_end_io is just ignored. 2) Even if it was real bh attached to a page, for DAX we don't do any writeback and thus ->b_end_io will never get called? 3) And if it does get called, you certainly cannot call ext4_convert_unwritten_extents() from softirq context where ->b_end_io gets called. This got added to fix a problem that Dave Chinner pointed out. We need the allocated extent to either be zeroed (as ext2 does), or marked as unwritten (ext4, XFS) so that a racing read/page fault doesn't return uninitialized data. If it's marked as unwritten, we need to convert it to a written extent after we've initialised the contents. We use the b_end_io() callback to do this, and it's called from the DAX code, not in softirq context. > if (io_end && io_end->flag & EXT4_IO_END_UNWRITTEN) > set_buffer_defer_completion(bh); > bh->b_size = inode->i_sb->s_blocksize * map.m_len; Honza -- 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
