linux-ext4 - Re: [PATCH 4/5] dax: use sb_issue_zerout instead of calling dax_clear

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Date:	Mon, 28 Mar 2016 20:01:29 +0000
From:	"Verma, Vishal L" <vishal.l.verma@...el.com>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
CC:	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"axboe@...com" <axboe@...com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	"david@...morbit.com" <david@...morbit.com>,
	"jack@...e.cz" <jack@...e.cz>
Subject: Re: [PATCH 4/5] dax: use sb_issue_zerout instead of calling
 dax_clear_sectors

On Fri, 2016-03-25 at 14:20 -0700, Dan Williams wrote:
> On Fri, Mar 25, 2016 at 2:03 PM, Verma, Vishal L
> <vishal.l.verma@...el.com> wrote:
> > 
> > On Fri, 2016-03-25 at 11:47 -0700, Dan Williams wrote:
> > > 
> > > On Thu, Mar 24, 2016 at 4:17 PM, Vishal Verma <vishal.l.verma@int
> > > el.c
> > > om> wrote:
> > > > 
> > > > 
> > > > From: Matthew Wilcox <matthew.r.wilcox@...el.com>
> > > > 
> > > > dax_clear_sectors() cannot handle poisoned blocks.  These must
> > > > be
> > > > zeroed using the BIO interface instead.  Convert ext2 and XFS
> > > > to
> > > > use
> > > > only sb_issue_zerout().
> > > > 
> > > > Signed-off-by: Matthew Wilcox <matthew.r.wilcox@...el.com>
> > > > [vishal: Also remove the dax_clear_sectors function entirely]
> > > > Signed-off-by: Vishal Verma <vishal.l.verma@...el.com>
> > > > ---
> > > >  fs/dax.c               | 32 --------------------------------
> > > >  fs/ext2/inode.c        |  7 +++----
> > > >  fs/xfs/xfs_bmap_util.c |  9 ---------
> > > >  include/linux/dax.h    |  1 -
> > > >  4 files changed, 3 insertions(+), 46 deletions(-)
> > > > 
> > > > diff --git a/fs/dax.c b/fs/dax.c
> > > > index bb7e9f8..a30481e 100644
> > > > --- a/fs/dax.c
> > > > +++ b/fs/dax.c
> > > > @@ -78,38 +78,6 @@ struct page *read_dax_sector(struct
> > > > block_device
> > > > *bdev, sector_t n)
> > > >         return page;
> > > >  }
> > > > 
> > > > -/*
> > > > - * dax_clear_sectors() is called from within transaction
> > > > context
> > > > from XFS,
> > > > - * and hence this means the stack from this point must follow
> > > > GFP_NOFS
> > > > - * semantics for all operations.
> > > > - */
> > > > -int dax_clear_sectors(struct block_device *bdev, sector_t
> > > > _sector,
> > > > long _size)
> > > > -{
> > > > -       struct blk_dax_ctl dax = {
> > > > -               .sector = _sector,
> > > > -               .size = _size,
> > > > -       };
> > > > -
> > > > -       might_sleep();
> > > > -       do {
> > > > -               long count, sz;
> > > > -
> > > > -               count = dax_map_atomic(bdev, &dax);
> > > > -               if (count < 0)
> > > > -                       return count;
> > > > -               sz = min_t(long, count, SZ_128K);
> > > > -               clear_pmem(dax.addr, sz);
> > > > -               dax.size -= sz;
> > > > -               dax.sector += sz / 512;
> > > > -               dax_unmap_atomic(bdev, &dax);
> > > > -               cond_resched();
> > > > -       } while (dax.size);
> > > > -
> > > > -       wmb_pmem();
> > > > -       return 0;
> > > > -}
> > > > -EXPORT_SYMBOL_GPL(dax_clear_sectors);
> > > What about the other unwritten extent conversions in the dax
> > > path?
> > > Shouldn't those be converted to block-layer zero-outs as well?
> > Could you point me to where these might be? I thought once we've
> > converted all the zeroout type callers (by removing
> > dax_clear_sectors),
> > and fixed up dax_do_io to try a driver fallback, we've handled all
> > the
> > media error cases in dax..
> grep for usages of clear_pmem()... which I was hoping to eliminate
> after this change to push zeroing down to the driver.

Ok, so I looked at these, and it looks like the majority of callers of
clear_pmem are from the fault path (either pmd or regular), and in
those cases we should be 'protected', as we would have failed at a
prior step (dax_map_atomic).

The two cases that may not be well handled are the calls to
dax_zero_page_range and dax_truncate_page which are called from file
systems. I think we may need to do a fallback to the driver for those
cases just like we do for dax_direct_io.. Thoughts?