linux-mm.kvack.org archive mirror
 help / color / mirror / Atom feed
From: Jan Kara <jack@suse.cz>
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: 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>, Jan Kara <jack@suse.cz>,
	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: Wed, 19 Jul 2017 17:33:14 +0200	[thread overview]
Message-ID: <20170719153314.GC15908@quack2.suse.cz> (raw)
In-Reply-To: <20170628220152.28161-4-ross.zwisler@linux.intel.com>

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.
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?

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

  reply	other threads:[~2017-07-19 15:33 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 [this message]
2017-07-19 16:26     ` Ross Zwisler
2017-07-20 10:27       ` Jan Kara
2017-07-20 14:28         ` Ross Zwisler
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20170719153314.GC15908@quack2.suse.cz \
    --to=jack@suse.cz \
    --cc=adilger.kernel@dilger.ca \
    --cc=akpm@linux-foundation.org \
    --cc=corbet@lwn.net \
    --cc=dan.j.williams@intel.com \
    --cc=darrick.wong@oracle.com \
    --cc=dave.hansen@intel.com \
    --cc=hch@lst.de \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-ext4@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-nvdimm@lists.01.org \
    --cc=linux-xfs@vger.kernel.org \
    --cc=mawilcox@microsoft.com \
    --cc=mingo@redhat.com \
    --cc=ross.zwisler@linux.intel.com \
    --cc=rostedt@goodmis.org \
    --cc=tytso@mit.edu \
    --cc=viro@zeniv.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).