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Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

From Dan Williams <dan.j.williams@intel.com>
Newsgroups linux.kernel
Subject Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences
Date 2016-02-02 18:50 +0100
Message-ID <qXNft-6ut-11@gated-at.bofh.it> (permalink)
References (6 earlier) <qXHa1-1VJ-13@gated-at.bofh.it> <qXM9J-5Km-21@gated-at.bofh.it> <qXMjo-5NR-23@gated-at.bofh.it> <qXMMr-6gE-9@gated-at.bofh.it> <qXN5O-6qA-57@gated-at.bofh.it>
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On Tue, Feb 2, 2016 at 9:34 AM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Tue, Feb 02, 2016 at 09:10:24AM -0800, Dan Williams wrote:
>> On Tue, Feb 2, 2016 at 8:46 AM, Jan Kara <jack@suse.cz> wrote:
>> > On Tue 02-02-16 08:33:56, Dan Williams wrote:
>> >> On Tue, Feb 2, 2016 at 3:17 AM, Jan Kara <jack@suse.cz> wrote:
>> >> [..]
>> >> > I see, thanks for explanation. So I'm OK with changing what is stored in
>> >> > the radix tree to accommodate this use case but my reservation that we IHMO
>> >> > have other more pressing things to fix remains...
>> >>
>> >> We don't need pfns in the radix to support XFS RT configurations.
>> >> Just call get_blocks() again and use the sector, or am I missing
>> >> something?
>> >
>> > You are correct. But if you decide to pay the cost of additional
>> > get_block() call, you only need the dirty tag in the radix tree and nothing
>> > else. So my understanding was that the whole point of games with radix tree
>> > is avoiding this extra get_block() calls for fsync().
>> >
>>
>> DAX-fsync() is already a potentially expensive operation to cover data
>> durability guarantees for DAX-unaware applications.  A DAX-aware
>> application is going to skip fsync, and the get_blocks() cost, to do
>> cache management itself.
>>
>> Willy pointed out some other potential benefits, assuming a suitable
>> replacement for the protections afforded by the block-device
>> percpu_ref counter can be found.  However, optimizing for the
>> DAX-unaware-application case seems the wrong motivation to me.
>
> Oh, no, the primary issue with calling get_block() in the fsync path isn't
> performance.  It's that we don't have any idea what get_block() function to
> call.
>
> The fault handler calls all come from the filesystem directly, so they can
> easily give us an appropriate get_block() function pointer.  But the
> dax_writeback_mapping_range() calls come from the generic code in
> mm/filemap.c, and don't know what get_block() to pass in.
>
> During one iteration I had the calls to dax_writeback_mapping_range()
> happening in the filesystem fsync code so that it could pass in get_block(),
> but Dave Chinner pointed out that this misses other paths in the filesystem
> that need to have things flushed via a call to filemap_write_and_wait_range().
>
> In yet another iteration of this series I tried adding get_block() to struct
> inode_operations so that I could access it from what is now
> dax_writeback_mapping_range(), but this was shot down as well.

Ugh, and we can't trigger it from where a filesystem normally syncs a
block device, becauDid you tryse we lose track of the inode radix
information at that level.

What a about a super_operation?  That seems the right level, given
we're currently doing:

inode->i_sb->s_bdev

...it does not seem terrible to instead do:

inode->i_sb->s_op->get_block()

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Thread

[PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-01-28 20:40 +0100
  Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-28 21:30 +0100
  Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Christoph Hellwig <hch@infradead.org> - 2016-01-28 22:40 +0100
    Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-01-29 19:30 +0100
      Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-01-30 00:40 +0100
        Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-30 01:20 +0100
        Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dave Chinner <david@fromorbit.com> - 2016-01-31 23:50 +0100
      Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-01-30 06:30 +0100
        Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-30 07:10 +0100
          Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jared Hulbert <jaredeh@gmail.com> - 2016-01-30 08:10 +0100
          Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-01-31 03:40 +0100
            Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <zwisler@gmail.com> - 2016-01-31 07:20 +0100
              Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-01-31 12:00 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-31 17:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-01-31 19:10 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-31 19:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-01-31 19:30 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-31 20:00 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-01-31 21:00 +0100
        Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-01 16:00 +0100
          Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-02-01 21:50 +0100
          Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dave Chinner <david@fromorbit.com> - 2016-02-01 22:50 +0100
            Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jared Hulbert <jaredeh@gmail.com> - 2016-02-02 07:10 +0100
              Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 07:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jared Hulbert <jaredeh@gmail.com> - 2016-02-02 09:10 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 18:00 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jared Hulbert <jaredeh@gmail.com> - 2016-02-02 22:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-02-03 01:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jared Hulbert <jaredeh@gmail.com> - 2016-02-03 02:30 +0100
            Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-02 12:20 +0100
              Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 17:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-02 17:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 18:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-02 18:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 18:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-02 19:30 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 18:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-02-02 19:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dan Williams <dan.j.williams@intel.com> - 2016-02-02 20:10 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Matthew Wilcox <willy@linux.intel.com> - 2016-02-02 21:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-03 12:10 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-03 11:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-03 21:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-04 10:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-05 00:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dave Chinner <david@fromorbit.com> - 2016-02-07 00:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-07 06:30 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-04 21:00 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-04 21:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-04 23:20 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-05 23:30 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dave Chinner <david@fromorbit.com> - 2016-02-07 00:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-07 07:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-08 14:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Christoph Hellwig <hch@infradead.org> - 2016-02-07 09:40 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-08 17:00 +0100
              Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-02 19:50 +0100
                Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-02 20:00 +0100
    Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-02-02 01:10 +0100
      Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Dave Chinner <david@fromorbit.com> - 2016-02-02 08:20 +0100
      Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences Jan Kara <jack@suse.cz> - 2016-02-02 11:40 +0100

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