linux-ext4 - Re: [PATCH v2 3/3] dax: use common 4k zero page for dax mmap reads

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Date:   Fri, 16 Jun 2017 13:45:40 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-doc@...r.kernel.org, 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 v2 3/3] dax: use common 4k zero page for dax mmap reads

On Thu, Jun 15, 2017 at 04:58:56PM +0200, Jan Kara wrote:
> On Wed 14-06-17 11:22:11, Ross Zwisler wrote:
> > @@ -216,17 +217,6 @@ static void dax_unlock_mapping_entry(struct address_space *mapping,
> >  	dax_wake_mapping_entry_waiter(mapping, index, entry, false);
> >  }
> >  
> > -static void put_locked_mapping_entry(struct address_space *mapping,
> > -				     pgoff_t index, void *entry)
> > -{
> > -	if (!radix_tree_exceptional_entry(entry)) {
> > -		unlock_page(entry);
> > -		put_page(entry);
> > -	} else {
> > -		dax_unlock_mapping_entry(mapping, index);
> > -	}
> > -}
> > -
> 
> The naming becomes asymetric with this. So I'd prefer keeping
> put_locked_mapping_entry() as a trivial wrapper around
> dax_unlock_mapping_entry() unless we can craft more sensible naming / API
> for entry grabbing (and that would be a separate patch anyway).

Sure, that works for me.  I'll fix for v3.

> > -static int dax_load_hole(struct address_space *mapping, void **entry,
> > +static int dax_load_hole(struct address_space *mapping, void *entry,
> >  			 struct vm_fault *vmf)
> >  {
> >  	struct inode *inode = mapping->host;
> > -	struct page *page;
> > -	int ret;
> > -
> > -	/* Hole page already exists? Return it...  */
> > -	if (!radix_tree_exceptional_entry(*entry)) {
> > -		page = *entry;
> > -		goto finish_fault;
> > -	}
> > +	unsigned long vaddr = vmf->address;
> > +	int ret = VM_FAULT_NOPAGE;
> > +	struct page *zero_page;
> > +	void *entry2;
> >  
> > -	/* This will replace locked radix tree entry with a hole page */
> > -	page = find_or_create_page(mapping, vmf->pgoff,
> > -				   vmf->gfp_mask | __GFP_ZERO);
> 
> With this gone, you can also remove the special DAX handling from
> mm/filemap.c: page_cache_tree_insert() and remove from dax.h
> dax_wake_mapping_entry_waiter(), dax_radix_locked_entry() and RADIX_DAX
> definitions. Yay! As a separate patch please.

Oh, yay!  :)  Sure, I'll have this patch for v3.