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Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
 pressure
To:     SeongJae Park <sjpark@amazon.com>, axboe@kernel.dk,
        konrad.wilk@oracle.com, roger.pau@citrix.com
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        pdurrant@amazon.com, sj38.park@gmail.com,
        xen-devel@lists.xenproject.org
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From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <954f7beb-9d40-253e-260b-4750809bf808@suse.com>
Date:   Mon, 9 Dec 2019 10:39:02 +0100
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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.


Juergen