RE: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory pressure

[Durrant, Paul]

> -----Original Message-----
> From: JÃrgen Groà <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-
> devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
> pressure
> 
> 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
> before.
> 
> 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
> pages.
> 
> 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.

  Paul