Re: [PATCH V18 01/18] Btrfs: subpage-blocksize: Fix whole page read.

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On Tuesday 26 Apr 2016 11:51:22 Josef Bacik wrote:
> > +int set_page_extent_mapped(struct page *page)
> > 
> >  {
> > 
> > +	struct btrfs_page_private *pg_private;
> > +
> > 
> >  	if (!PagePrivate(page)) {
> > 
> > +		pg_private = kzalloc(sizeof(*pg_private), GFP_NOFS);
> > +		if (!pg_private)
> > +			return -ENOMEM;
> 
> So I would like to avoid the per-page allocation in the case that
> sectorsize == pagesize.  Also this is going to be pretty heavily used,
> so a separate slab should be used.

Ok. I will revise this patch and other dependent patches to determine the
block states (dirty, uptodate, etc) from either page flags (for sectorsize ==
pagesize) or from the per-page bitmap (for sectorsize < pagesize).

> 
> In fact, couldn't we just use the extent io tree to deal with this?  It
> would be kind of heavy handed to have to look up in the io tree for
> every sub page range I suppose, but we could probably avoid doing it in
> most cases and only in the case that we know we're only doing the sub
> page IO.  Thanks,
>

Commit 1edbb734b4e010974c41d2859d22a43d04f5f1cf (Btrfs: reduce CPU usage in
the extent_state tree) dropped support for tracking block states using extent
io tree citing performance reasons. The initial versions of subpage-blocksize
patchset did bring back this feature. However during Btrfs BoF meetup during
2015 Vault conference, we decided to go with the per-page bitmap to track the
block states.

-- 
chandan