Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences
From: Jared Hulbert
Date: Tue Feb 02 2016 - 20:22:00 EST
On Tue, Feb 2, 2016 at 4:34 PM, Matthew Wilcox <willy@xxxxxxxxxxxxxxx> wrote:
> On Tue, Feb 02, 2016 at 01:46:06PM -0800, Jared Hulbert wrote:
>> On Tue, Feb 2, 2016 at 8:51 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> >> The filesystem I'm concerned with is AXFS
>> >> (https://www.kernel.org/doc/ols/2008/ols2008v1-pages-211-218.pdf).
>> >> Which I've been planning on trying to merge again due to a recent
>> >> resurgence of interest. The device model for AXFS is... weird. It
>> >> can use one or two devices at a time of any mix of NOR MTD, NAND MTD,
>> >> block, and unmanaged physical memory. It's a terribly useful model
>> >> for embedded. Anyway AXFS is readonly so hacking in a read only
>> >> dax_fault_nodev() and dax_file_read() would work fine, looks easy
>> >> enough. But... it would be cool if similar small embedded focused RW
>> >> filesystems were enabled.
>> >
>> > Are those also out of tree?
>>
>> Of course. Merging embedded filesystems is little merging regular
>> filesystems except 98% of you reviewers don't want it merged.
>
> You should at least be able to get it into staging these days. I mean,
> look at some of the junk that's in staging ... and I don't think AXFS was
> nearly as bad.
Thanks....? ;)
>> IMO you're making DAX more complex by overly coupling to the bdev and
>> I think it could bite you later. I submit this rework of the radix
>> tree and confusion about where to get the real bdev as evidence. I'm
>> guessing that it won't be the last time. It's unnecessary to couple
>> it like this, and in fact is not how the vfs has been layered in the
>> past.
>
> Huh? The rework to use the radix tree for PFNs was done with one eye
> firmly on your usage case. Just because I had to thread the get_block
> interface through it for the moment doesn't mean that I didn't have
> the "how do we get rid of get_block entirely" question on my mind.
Oh yeah. I think we're on the same page. But I'm not sure Dan is. I
get the need to phase this in too.
> Using get_block seemed like the right idea three years ago. I didn't
> know just how fundamentally ext4 and XFS disagree on how it should be
> used.
Sure. I can see that.
>> To look at the the downside consider dax_fault(). Its called on a
>> fault to a user memory map, uses the filesystems get_block() to lookup
>> a sector so you can ask a block device to convert it to an address on
>> a DIMM. Come on, that's awkward. Everything around dax_fault() is
>> dripping with memory semantic interfaces, the dax_fault() call are
>> fundamentally about memory, the pmem calls are memory, the hardware is
>> memory, and yet it directly calls bdev_direct_access(). It's out of
>> place.
>
> What was out of place was the old 'get_xip_mem' in address_space
> operations. Returning a kernel virtual address and a PFN from a
> filesystem operation? That looks awful.
Yes. Yes it does! But at least my big hack was just one line. ;)
Nobody really even seemed to notice at the time.
> All the other operations deal
> in struct pages, file offsets and occasionally sectors. Of course, we
> don't have a struct page, so a pfn makes sense, but the kernel virtual
> address being returned was a gargantuan layering problem.
Well yes, but it was an expedient hack.
>> The legacy vfs/mm code didn't have this layering problem either. Even
>> filemap_fault() that dax_fault() is modeled after doesn't call any
>> bdev methods directly, when it needs something it asks the filesystem
>> with a ->readpage(). The precedence is that you ask the filesystem
>> for what you need. Look at the get_bdev() thing you've concluded you
>> need. It _almost_ makes my point. I just happen to be of the opinion
>> that you don't actually want or need the bdev, you want the pfn/kaddr
>> so you can flush or map or memcpy().
>
> You want the pfn. The device driver doesn't have enough information to
> give you a (coherent with userspace) kaddr. That's what (some future
> arch-specific implementation of) dax_map_pfn() is for. That's why it
> takes 'index' as a parameter, so you can calculate where it'll be mapped
> in userspace, and determine an appropriate kernel virtual address to
> use for it.
Oh.... I think I'm just beginning to catch your vision for
dax_map_pfn(). I still don't get why we can't just do semi-arch
specific flushing instead of the alignment thing. But that just might
be epic ignorance on my part. Either way flush or magic alignments
dax_(un)map_pfn() would handle it, right?