From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Jan Kara <jack@suse.cz>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org,
"Darrick J. Wong" <darrick.wong@oracle.com>,
"Theodore Ts'o" <tytso@mit.edu>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Christoph Hellwig <hch@lst.de>,
Dan Williams <dan.j.williams@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Ingo Molnar <mingo@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
Matthew Wilcox <mawilcox@microsoft.com>,
Steven Rostedt <rostedt@goodmis.org>,
linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-nvdimm@lists.01.org, linux-xfs@vger.kernel.org
Subject: Re: [PATCH v3 3/5] dax: use common 4k zero page for dax mmap reads
Date: Thu, 20 Jul 2017 08:28:40 -0600 [thread overview]
Message-ID: <20170720142840.GB20414@linux.intel.com> (raw)
In-Reply-To: <20170720102723.GB17689@quack2.suse.cz>
On Thu, Jul 20, 2017 at 12:27:23PM +0200, Jan Kara wrote:
> On Wed 19-07-17 10:26:45, Ross Zwisler wrote:
> > On Wed, Jul 19, 2017 at 05:33:14PM +0200, Jan Kara wrote:
> > > On Wed 28-06-17 16:01:50, Ross Zwisler wrote:
> > > > Another major change is that we remove dax_pfn_mkwrite() from our fault
> > > > flow, and instead rely on the page fault itself to make the PTE dirty and
> > > > writeable. The following description from the patch adding the
> > > > vm_insert_mixed_mkwrite() call explains this a little more:
> > > >
> > > > ***
> > > > To be able to use the common 4k zero page in DAX we need to have our PTE
> > > > fault path look more like our PMD fault path where a PTE entry can be
> > > > marked as dirty and writeable as it is first inserted, rather than
> > > > waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
> > > >
> > > > Right now we can rely on having a dax_pfn_mkwrite() call because we can
> > > > distinguish between these two cases in do_wp_page():
> > > >
> > > > case 1: 4k zero page => writable DAX storage
> > > > case 2: read-only DAX storage => writeable DAX storage
> > > >
> > > > This distinction is made by via vm_normal_page(). vm_normal_page()
> > > > returns false for the common 4k zero page, though, just as it does for
> > > > DAX ptes. Instead of special casing the DAX + 4k zero page case, we will
> > > > simplify our DAX PTE page fault sequence so that it matches our DAX PMD
> > > > sequence, and get rid of dax_pfn_mkwrite() completely.
> > > >
> > > > This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
> > > > and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set
> > > > insert_pfn() will do the work that was previously done by wp_page_reuse()
> > > > as part of the dax_pfn_mkwrite() call path.
> > > > ***
> > >
> > > Hum, thinking about this in context of this patch... So what if we have
> > > allocated storage, a process faults it read-only, we map it to page tables
> > > writeprotected. Then the process writes through mmap to the area - the code
> > > in handle_pte_fault() ends up in do_wp_page() if I'm reading it right.
> >
> > Yep.
> >
> > > Then, since we are missing ->pfn_mkwrite() handlers, the PTE will be marked
> > > writeable but radix tree entry stays clean - bug. Am I missing something?
> >
> > I don't think we ever end up with a writeable PTE but with a clean radix tree
> > entry. When we get the write fault we do a full fault through
> > dax_iomap_pte_fault() and dax_insert_mapping().
> >
> > dax_insert_mapping() sets up the dirty radix tree entry via
> > dax_insert_mapping_entry() before it does anything with the page tables via
> > vm_insert_mixed_mkwrite().
> >
> > So, this mkwrite fault path is exactly the path we would have taken if the
> > initial read to real storage hadn't happened, and we end up in the same end
> > state - with a dirty DAX radix tree entry and a writeable PTE.
>
> Ah sorry, I have missed that it is not that you would not have
> ->pfn_mkwrite() handler - you still have it but it is the same as standard
> fault handler now. So maybe can you rephrase the changelog a bit saying
> that: "We get rid of dax_pfn_mkwrite() helper and use dax_iomap_fault() to
> handle write-protection faults instead." Thanks!
Ah, sure, will do.
next prev parent reply other threads:[~2017-07-20 14:28 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-28 22:01 [PATCH v3 0/5] DAX common 4k zero page Ross Zwisler
2017-06-28 22:01 ` [PATCH v3 1/5] mm: add vm_insert_mixed_mkwrite() Ross Zwisler
2017-07-19 14:16 ` Jan Kara
2017-07-19 17:51 ` Ross Zwisler
2017-07-19 21:58 ` Ross Zwisler
2017-07-21 17:44 ` Ross Zwisler
2017-07-24 11:20 ` Jan Kara
2017-07-20 15:26 ` Vivek Goyal
2017-07-20 15:59 ` Ross Zwisler
2017-07-21 18:02 ` Ross Zwisler
2017-06-28 22:01 ` [PATCH v3 2/5] dax: relocate some dax functions Ross Zwisler
2017-06-28 22:01 ` [PATCH v3 3/5] dax: use common 4k zero page for dax mmap reads Ross Zwisler
2017-07-19 15:33 ` Jan Kara
2017-07-19 16:26 ` Ross Zwisler
2017-07-20 10:27 ` Jan Kara
2017-07-20 14:28 ` Ross Zwisler [this message]
2017-06-28 22:01 ` [PATCH v3 4/5] dax: remove DAX code from page_cache_tree_insert() Ross Zwisler
2017-06-28 22:01 ` [PATCH v3 5/5] dax: move all DAX radix tree defs to fs/dax.c Ross Zwisler
2017-06-30 19:05 ` [PATCH v3 0/5] DAX common 4k zero page Ross Zwisler
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