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Re: [Xen-devel] [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory pressure

On 09.12.19 11:52, SeongJae Park wrote:
On Mon, 9 Dec 2019 11:15:22 +0100 "Jürgen Groß" <jgross@xxxxxxxx> wrote:

On 09.12.19 10:46, Durrant, Paul wrote:
-----Original Message-----
From: Jürgen Groß <jgross@xxxxxxxx>
Sent: 09 December 2019 09:39
To: Park, Seongjae <sjpark@xxxxxxxxxx>; axboe@xxxxxxxxx;
konrad.wilk@xxxxxxxxxx; roger.pau@xxxxxxxxxx
Cc: linux-block@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Durrant,
Paul <pdurrant@xxxxxxxxxx>; sj38.park@xxxxxxxxx; xen-
Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory

On 09.12.19 09:58, SeongJae Park wrote:
Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and be increased on demand while processing
the I/O requests.  If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.

Therefore, `blkfront` running guests can cause a memory pressure in the
`blkback` running guest by attaching a large number of block devices and
inducing I/O.

I'm having problems to understand how a guest can attach a large number
of block devices without those having been configured by the host admin

If those devices have been configured, dom0 should be ready for that
number of devices, e.g. by having enough spare memory area for ballooned

So either I'm missing something here or your reasoning for the need of
the patch is wrong.

I think the underlying issue is that persistent grant support is hogging memory 
in the backends, thereby compromising scalability. IIUC this patch is 
essentially a band-aid to get back to the scalability that was possible before 
persistent grant support was added. Ultimately the right answer should be to 
get rid of persistent grants support and use grant copy, but such a change is 
clearly more invasive and would need far more testing.

Persistent grants are hogging ballooned pages, which is equivalent to
memory only in case of the backend's domain memory being equal or
rather near to its max memory size.

So configuring the backend domain with enough spare area for ballooned
pages should make this problem much less serious.

Another problem in this area is the amount of maptrack frames configured
for a driver domain, which will limit the number of concurrent foreign
mappings of that domain.

Right, similar problems from other backends are possible.

So instead of having a blkback specific solution I'd rather have a
common callback for backends to release foreign mappings in order to
enable a global resource management.

This patch is also based on a common callback, namely the shrinker callback
system.  As the shrinker callback is designed for the general memory pressure
handling, I thought this is a right one to use.  Other backends having similar
problems can use this in their way.

But this is addressing memory shortage only and it is acting globally.

What I'd like to have in some (maybe distant) future is a way to control
resource usage per guest. Why would you want to throttle performance of
all guests instead of only the one causing the pain by hogging lots of

The new backend callback should (IMO) have a domid as parameter for
specifying which guest should be taken away resources (including the
possibility to select "any domain").

It might be reasonable to have your shrinker hook in e.g. xenbus for
calling the backend callbacks. And you could have another agent in the
grant driver reacting on shortage of possible grant mappings.

I don't expect you to implement all of that at once, but I think having
that idea in mind when addressing current issues would be nice. So as a
starting point you could move the shrinker hook to xenbus, add the
generic callback to struct xenbus_driver, populate that callback in
blkback and call it in the shrinker hook with "any domain". This would
enable a future extension to other backends and a dynamic resource
management in a natural way.


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