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

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Date:	Tue, 2 Feb 2016 09:47:37 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

On Tue, Feb 2, 2016 at 9:46 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Tue, Feb 2, 2016 at 9:34 AM, Ross Zwisler
> <ross.zwisler@...ux.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@...e.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@...e.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

[ sorry, copy paste error ]

block device, because 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()