linux-ext4 - Re: [PATCH v4 07/12] dax: coordinate locking for offsets in PMD range

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Date:   Mon, 3 Oct 2016 12:40:02 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH v4 07/12] dax: coordinate locking for offsets in PMD range

On Mon, Oct 03, 2016 at 11:55:18AM +0200, Jan Kara wrote:
> On Thu 29-09-16 16:49:25, Ross Zwisler wrote:
> > DAX radix tree locking currently locks entries based on the unique
> > combination of the 'mapping' pointer and the pgoff_t 'index' for the entry.
> > This works for PTEs, but as we move to PMDs we will need to have all the
> > offsets within the range covered by the PMD to map to the same bit lock.
> > To accomplish this, for ranges covered by a PMD entry we will instead lock
> > based on the page offset of the beginning of the PMD entry.  The 'mapping'
> > pointer is still used in the same way.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > ---
> >  fs/dax.c            | 37 ++++++++++++++++++++++++-------------
> >  include/linux/dax.h |  2 +-
> >  mm/filemap.c        |  2 +-
> >  3 files changed, 26 insertions(+), 15 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index baef586..406feea 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -64,10 +64,17 @@ static int __init init_dax_wait_table(void)
> >  }
> >  fs_initcall(init_dax_wait_table);
> >  
> > +static pgoff_t dax_entry_start(pgoff_t index, void *entry)
> > +{
> > +	if (RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
> > +		index &= (PMD_MASK >> PAGE_SHIFT);
> 
> Hum, but if we shift right, top bits of PMD_MASK will become zero - not
> something we want I guess... You rather want to mask with something like:
> 	~((1UL << (PMD_SHIFT - PAGE_SHIFT)) - 1)

Great catch, thanks.  Fixed for v5.

> > @@ -447,10 +457,11 @@ restart:
> >  	return entry;
> >  }
> >  
> > -void dax_wake_mapping_entry_waiter(struct address_space *mapping,
> > +void dax_wake_mapping_entry_waiter(void *entry, struct address_space *mapping,
> >  				   pgoff_t index, bool wake_all)
> 
> Nitpick: Ordering of arguments would look more logical to me like:
> 
> dax_wake_mapping_entry_waiter(mapping, index, entry, wake_all)

Cool, I'll reorder them for v5.

Thanks for the review!
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