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Fri, 30 Nov 2018 22:33:49 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id wAUMXmfk008851 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 30 Nov 2018 22:33:48 GMT Received: from abhmp0001.oracle.com (abhmp0001.oracle.com [141.146.116.7]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id wAUMXmlt013335; Fri, 30 Nov 2018 22:33:48 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 30 Nov 2018 14:33:48 -0800 Subject: Re: [Xen-devel] [PATCH] xen-blkfront: use old rinfo after enomem during migration To: Manjunath Patil , jgross@suse.com, konrad.wilk@oracle.com, roger.pau@citrix.com, axboe@kernel.dk Cc: linux-block@vger.kernel.org, xen-devel@lists.xenproject.org References: <1543468665-22795-1-git-send-email-manjunath.b.patil@oracle.com> <1dafcf3d-c3b6-e6c5-f5d4-fbdb549aaa9c@oracle.com> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0 MIME-Version: 1.0 In-Reply-To: <1dafcf3d-c3b6-e6c5-f5d4-fbdb549aaa9c@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9093 signatures=668686 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=2 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1811300190 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 11/30/18 4:49 PM, Manjunath Patil wrote: > Thank you Boris for your comments. I removed faulty email of mine. > > replies inline. > On 11/30/2018 12:42 PM, Boris Ostrovsky wrote: >> On 11/29/18 12:17 AM, Manjunath Patil wrote: >>> Hi, >>> Feel free to suggest/comment on this. >>> >>> I am trying to do the following at dst during the migration now. >>> 1. Dont clear the old rinfo in blkif_free(). Instead just clean it. >>> 2. Store the old rinfo and nr_rings into temp variables in >>> negotiate_mq() >>> 3. let nr_rings get re-calculated based on backend data >>> 4. try allocating new memory based on new nr_rings >> Since I suspect number of rings will likely be the same why not reuse >> the rings in the common case? > I thought attaching devices will be more often than migration. Hence > did not want add to an extra check for >   - if I am inside migration code path and >   - if new nr_rings is equal to old nr_rings or not > > Sure addition of such a thing would avoid the memory allocation > altogether in migration path, > but it would add a little overhead for normal device addition. > > Do you think its worth adding that change? IMO a couple of extra checks are not going to make much difference. I wonder though --- have you actually seen the case where you did fail allocation and changes provided in this patch made things work? I am asking because right after negotiate_mq() we will call setup_blkring() and it will want to allocate bunch of memory. A failure there is fatal (to ring setup). So it seems to me that you will survive negotiate_mq() but then will likely fail soon after. >> >> >>> 5. >>>    a. If memory allocation is a success >>>       - free the old rinfo and proceed to use the new rinfo >>>    b. If memory allocation is a failure >>>       - use the old the rinfo >>>       - adjust the nr_rings to the lowest of new nr_rings and old >>> nr_rings >> >>> @@ -1918,10 +1936,24 @@ static int negotiate_mq(struct blkfront_info >>> *info) >>>                     sizeof(struct blkfront_ring_info), >>>                     GFP_KERNEL); >>>       if (!info->rinfo) { >>> -        xenbus_dev_fatal(info->xbdev, -ENOMEM, "allocating >>> ring_info structure"); >>> -        info->nr_rings = 0; >>> -        return -ENOMEM; >>> -    } >>> +        if (unlikely(nr_rings_old)) { >>> +            /* We might waste some memory if >>> +             * info->nr_rings < nr_rings_old >>> +             */ >>> +            info->rinfo = rinfo_old; >>> +            if (info->nr_rings > nr_rings_old) >>> +                info->nr_rings = nr_rings_old; >>> +            xenbus_dev_fatal(info->xbdev, -ENOMEM, >> >> Why xenbus_dev_fatal()? > I wanted to make sure that this msg is seen on console by default. So > that we know there was a enomem event happened and we recovered from it. > What do you suggest instead? xenbus_dev_error? Neither. xenbus_dev_fatal() is going to change connection state so it is certainly not what we want. And even xenbus_dev_error() doesn't look like the right thing to do since as far as block device setup is concerned there are no errors. Maybe pr_warn(). -boris >> >> -boris >> >> >>> +            "reusing old ring_info structure(new ring size=%d)", >>> +                info->nr_rings); >>> +        } else { >>> +            xenbus_dev_fatal(info->xbdev, -ENOMEM, >>> +                "allocating ring_info structure"); >>> +            info->nr_rings = 0; >>> +            return -ENOMEM; >>> +        } >>> +    } else if (unlikely(nr_rings_old)) >>> +        kfree(rinfo_old); >>>         for (i = 0; i < info->nr_rings; i++) { >>>           struct blkfront_ring_info *rinfo; >