linux-kernel - Re: [PATCH v3 3/7] mm: add find_get_entries

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Date:	Thu, 10 Dec 2015 13:24:38 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...il.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Dave Chinner <david@...morbit.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-mm <linux-mm@...ck.org>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	ext4 hackers <linux-ext4@...r.kernel.org>, xfs@....sgi.com,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>
Subject: Re: [PATCH v3 3/7] mm: add find_get_entries_tag()

On Wed, Dec 09, 2015 at 11:44:16AM -0800, Dan Williams wrote:
> On Tue, Dec 8, 2015 at 11:18 AM, Ross Zwisler
> <ross.zwisler@...ux.intel.com> wrote:
> > Add find_get_entries_tag() to the family of functions that include
> > find_get_entries(), find_get_pages() and find_get_pages_tag().  This is
> > needed for DAX dirty page handling because we need a list of both page
> > offsets and radix tree entries ('indices' and 'entries' in this function)
> > that are marked with the PAGECACHE_TAG_TOWRITE tag.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
<> 
> Why does this mostly duplicate find_get_entries()?
> 
> Surely find_get_entries() can be implemented as a special case of
> find_get_entries_tag().

I'm adding find_get_entries_tag() to the family of functions that already
exist and include find_get_entries(), find_get_pages(),
find_get_pages_contig() and find_get_pages_tag().

These functions all contain very similar code with small changes to the
internal looping based on whether you're looking through all radix slots or
only the ones that match a certain tag (radix_tree_for_each_slot() vs
radix_tree_for_each_tagged()).

We already have find_get_page() to get all pages in a range and
find_get_pages_tag() to get all pages in the range with a certain tag.  We
have find_get_entries() to get all pages and indices for a given range, but we
are currently missing find_get_entries_tag() to do that same search based on a
tag, which is what I'm adding.

I agree that we could probably figure out a way to combine the code for
find_get_entries() with find_get_entries_tag(), as we could do for the
existing functions find_get_pages() and find_get_pages_tag().  I think we
should probably add find_get_entries_tag() per this patch, though, and then
decide whether to do any combining later as a separate step.
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