On 12/13/2018 7:43 AM, Jerome Glisse wrote:
On Wed, Dec 12, 2018 at 08:20:43PM -0700, Jason Gunthorpe wrote:
On Wed, Dec 12, 2018 at 07:01:09PM -0500, Jerome Glisse wrote:
On Wed, Dec 12, 2018 at 04:37:03PM -0700, Jason Gunthorpe wrote:
On Wed, Dec 12, 2018 at 04:53:49PM -0500, Jerome Glisse wrote:
Almost, we need some safety around assuming that DMA is complete the
page, so the notification would need to go all to way to userspace
with something like a file lease notification. It would also need to
be backstopped by an IOMMU in the case where the hardware does not /
can not stop in-flight DMA.
You can always reprogram the hardware right away it will redirect
any dma to the crappy page.
That causes silent data corruption for RDMA users - we can't do that.
The only way out for current hardware is to forcibly terminate the
RDMA activity somehow (and I'm not even sure this is possible, at
least it would be driver specific)
Even the IOMMU idea probably doesn't work, I doubt all current
hardware can handle a PCI-E error TLP properly.
What i saying is reprogram hardware to crappy page ie valid page
dma map but that just has random content as a last resort to allow
filesystem to reuse block. So their should be no PCIE error unless
hardware freak out to see its page table reprogram randomly.
No, that isn't an option. You can't silently provide corrupted data
for RDMA to transfer out onto the network, or silently discard data
coming in!!
Think of the consequences of that - I have a fileserver process and
someone does ftruncate and now my clients receive corrupted data??
This is what happens _today_ ie today someone do GUP on page file
and then someone else do truncate the first GUP is effectively
streaming _random_ data to network as the page does not correspond
to anything anymore and once the RDMA MR goes aways and release
the page the page content will be lost. So i am not changing anything
here, what i proposed was to make it explicit to device driver at
least that they were streaming random data. Right now this is all
silent but this is what is happening wether you like it or not :)
Note that i am saying do that only for truncate to allow to be
nice to fs. But again i am fine with whatever solution but you can
not please everyone here. Either block truncate and fs folks will
hate you or make it clear to device driver that you are streaming
random things and RDMA people hates you.
The only option is to prevent the RDMA transfer from ever happening,
and we just don't have hardware support (beyond destroy everything) to
do that.
The question is who do you want to punish ? RDMA user that pin stuff
and expect thing to work forever without worrying for other fs
activities ? Or filesystem to pin block forever :)
I don't want to punish everyone, I want both sides to have complete
data integrity as the USER has deliberately decided to combine DAX and
RDMA. So either stop it at the front end (ie get_user_pages_longterm)
or make it work in a way that guarantees integrity for both.
S2: notify userspace program through device/sub-system
specific API and delay ftruncate. After a while if there
is no answer just be mean and force hardware to use
crappy page as anyway this is what happens today
I don't think this happens today (outside of DAX).. Does it?
It does it is just silent, i don't remember anything in the code
that would stop a truncate to happen because of elevated refcount.
This does not happen with ODP mlx5 as it does abide by _all_ mmu
notifier. This is for anything that does ODP without support for
mmu notifier.
Wait - is it expected that the MMU notifier upcall is handled
synchronously? That is, the page DMA mapping must be torn down
immediately, and before returning?
That's simply not possible, since the hardware needs to get control
to do this. Even if there were an IOMMU that could intercept the
DMA, reprogramming it will require a flush, which cannot be guaranteed
to occur "inline".
.. and the remedy here is to kill the process, not provide corrupt
data. Kill the process is likely to not go over well with any real
users that want this combination.
Think Samba serving files over RDMA - you can't have random unpriv
users calling ftruncate and causing smbd to be killed or serve corrupt
data.
So what i am saying is there is a choice and it would be better to
decide something than let the existing status quo where we just keep
streaming random data after truncate to a GUPed page.
Let's also remember that any torn-down DMA mapping can't be recycled
until all uses of the old DMA addresses are destroyed. The whole
thing screams for reference counting all the way down, to me.
Tom.
