Re: + ext4-add-dax-functionality.patch added to -mm tree

[Jan Kara <jack@suse.cz>]

  Matthew, I think I still didn't see response to this. I think we can
fixup things after they are merged (since Andrew sent this patch to Linus)
but IMHO it needs some action...

								Honza
On Mon 19-01-15 15:18:58, Jan Kara wrote:
> On Fri 16-01-15 21:16:03, Wilcox, Matthew R wrote:
> > 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.
>   Hum, so if I understand the code right dax_fault() will allocate a block
> (== page in persistent memory) for a faulted address and will map this
> block directly into process' address space. Thus that block has to be
> zeroed out before the fault finishes no matter what (so that userspace
> doesn't see garbage) - unwritten block handling in the filesystem doesn't
> really matter (and would only cause unnecessary overhead) because of the
> direct mapping of the block to process' address space. So I would think
> that it would be easiest if dax_fault() simply zeroed out blocks which got
> allocated. You could rewrite part of dax_fault() to something like:
> 
> 	create = !vmf->cow_page && (vmf->flags & FAULT_FLAG_WRITE);
> 	error = get_block(inode, block, &bh, create);
> 	if (!error && (bh.b_size < PAGE_SIZE))
> 		error = -EIO;
> 	if (error)
> 		goto unlock_page;
> 
> 	if (buffer_new(&bh)) {
> 		count_vm_event(PGMAJFAULT);
> 		mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
> 		major = VM_FAULT_MAJOR;
> 		dax_clear_blocks(inode, bh->b_blocknr, PAGE_SIZE);
> 	} else if (!buffer_mapped(&bh))
> 		return dax_load_hole(mapping, page, vmf);
> 
> Note, that we also avoided calling get_block() callback twice on major fault
> as that's relatively expensive due to locking, extent tree lookups, etc.
> 
> Also note that ext2 then doesn't have to call dax_clear_blocks() at all if
> I understand the code right.
> 
> And with this change to dax_fault() using ext4_get_block() is the right
> function to call. It will just allocate a block if necessary and attach it
> to an appropriate place in the extent tree which is all you need.
> 
> > > > @@ -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.
>   OK, I see. But I didn't find where ->b_end_io gets called from dax code
> (specifically I don't see it anywhere in dax_do_IO() or dax_io()). Can you
> point me please?
> 
> Also abusing b_end_io of a phony buffer for that looks ugly to me (we are
> trying to get away from passing phony bh around and this would entangle us
> even more into that mess). Normally I would think that end_io() callback
> passed into dax_do_io() should perform necessary conversions and for
> dax_fault() we could do necessary conversions inside foofs_page_mkwrite()...
> 
> 									Honza
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR