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Re: [PULL] vhost: cleanups and fixes




On 06/12/2018 09:59 AM, Linus Torvalds wrote:
On Mon, Jun 11, 2018 at 6:36 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
Maybe it will help to have GFP_NONE which will make any allocation
fail if attempted. Linus, would this address your comment?
It would definitely have helped me initially overlook that call chain.

But then when I started looking at the whole dma_map_page() thing, it
just raised my hackles again.

I would seriously suggest having a much simpler version for the "no
allocation, no dma mapping" case, so that it's *obvious* that that
never happens.

So instead of having virtio_balloon_send_free_pages() call a really
generic complex chain of functions that in _some_ cases can do memory
allocation, why isn't there a short-circuited "vitruque_add_datum()"
that is guaranteed to never do anything like that?

Honestly, I look at "add_one_sg()" and it really doesn't make me
happy. It looks hacky as hell. If I read the code right, you're really
trying to just queue up a simple tuple of <pfn,len>, except you encode
it as a page pointer in order to play games with the SG logic, and
then you hmap that to the ring, except in this case it's all a fake
ring that just adds the cpu-physical address instead.

And to figuer that out, it's like five layers of indirection through
different helper functions that *can* do more generic things but in
this case don't.

And you do all of this from a core VM callback function with some
_really_ core VM locks held.

That makes no sense to me.

How about this:

  - get rid of all that code

  - make the core VM callback save the "these are the free memory
regions" in a fixed and limited array. One that DOES JUST THAT. No
crazy "SG IO dma-mapping function crap". Just a plain array of a fixed
size, pre-allocated for that virtio instance.

  - make it obvious that what you do in that sequence is ten
instructions and no allocations ("Look ma, I wrote a value to an array
and incremented the array idex, and I'M DONE")

  - then in that workqueue entry that you start *anyway*, you empty the
array and do all the crazy virtio stuff.

In fact, while at it, just simplify the VM interface too. Instead of
traversing a random number of buddy lists, just trraverse *one* - the
top-level one. Are you seriously ever going to shrink or mark
read-only anythin *but* something big enough to be in the maximum
order?

MAX_ORDER is what, 11? So we're talking 8MB blocks. Do you *really*
want the balloon code to work on smaller things, particularly since
the whole interface is fundamentally racy and opportunistic to begin
with?

OK, I will implement a new version based on the suggestions. Thanks.

Best,
Wei