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Date: Wed, 23 Jul 2014 13:54:07 +0100
From: Alasdair G Kergon <agk@redhat.com>
To: Mikulas Patocka <mpatocka@redhat.com>,
        "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
        "xinhui.pan" <xinhuix.pan@intel.com>, linux-kernel@vger.kernel.org,
        agk@redhat.com, snitzer@redhat.com, dm-devel@redhat.com,
        "Liu, ShuoX" <shuox.liu@intel.com>
Subject: Re: [dm-devel] [PATCH] md/dm-ioctl.c: optimize memory allocation
	in copy_params
Message-ID: <20140723125407.GG6822@agk-dp.fab.redhat.com>
Mail-Followup-To: Mikulas Patocka <mpatocka@redhat.com>,
	"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
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	agk@redhat.com, snitzer@redhat.com, dm-devel@redhat.com,
	"Liu, ShuoX" <shuox.liu@intel.com>
References: <53B68068.2060102@intel.com> <alpine.LRH.2.02.1407081831510.27831@file01.intranet.prod.int.rdu2.redhat.com> <53BCA278.5050205@linux.intel.com> <alpine.LRH.2.02.1407091032400.4545@file01.intranet.prod.int.rdu2.redhat.com> <53CDB81B.90509@linux.intel.com> <20140722012352.GD6822@agk-dp.fab.redhat.com> <53CF26B2.6060503@linux.intel.com> <alpine.LRH.2.02.1407230816000.29551@file01.intranet.prod.int.rdu2.redhat.com>
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On Wed, Jul 23, 2014 at 08:16:58AM -0400, Mikulas Patocka wrote:
> So, it means that you do not use device mapper at all. So, why are you 
> trying to change memory allocation in device mapper?
 
So the *test* they run is asking device-mapper to briefly reserve a 64KB
buffer when there is no data to report:  The answer is not to run that
pointless test:)

And if a single 64KB allocation really is a big deal, then patch 'vold'
in userspace so it doesn't ask for 64KB when it clearly doesn't need it!

+ int Devmapper::dumpState(SocketClient *c) {
+    char *buffer = (char *) malloc(1024 * 64);

The code has just
#define DEVMAPPER_BUFFER_SIZE 4096
for all the other dm ioctls it issues.

Only use a larger value when it is needed i.e. if DM_BUFFER_FULL_FLAG gets set.

Alasdair