From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f41.google.com ([209.85.218.41]:36066 "EHLO mail-oi0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750999AbbLDRoV (ORCPT ); Fri, 4 Dec 2015 12:44:21 -0500 Received: by oiww189 with SMTP id w189so70907267oiw.3 for ; Fri, 04 Dec 2015 09:44:21 -0800 (PST) Subject: Re: [PATCH] block: detach bdev inode from its wb in __blkdev_put() To: Ilya Dryomov , Alexander Viro References: <1448054554-24138-1-git-send-email-idryomov@gmail.com> <20151130072029.GA10738@linux.vnet.ibm.com> Cc: Tejun Heo , Christoph Hellwig , linux-fsdevel@vger.kernel.org, Raghavendra K T From: Jens Axboe Message-ID: <5661D0F3.3080603@kernel.dk> Date: Fri, 4 Dec 2015 10:44:19 -0700 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 12/04/2015 07:07 AM, Ilya Dryomov wrote: > On Mon, Nov 30, 2015 at 11:54 AM, Ilya Dryomov wrote: >> On Mon, Nov 30, 2015 at 8:20 AM, Raghavendra K T >> wrote: >>> * Ilya Dryomov [2015-11-20 22:22:34]: >>> >>>> Since 52ebea749aae ("writeback: make backing_dev_info host >>>> cgroup-specific bdi_writebacks") inode, at some point in its lifetime, >>>> gets attached to a wb (struct bdi_writeback). Detaching happens on >>>> evict, in inode_detach_wb() called from __destroy_inode(), and involves >>>> updating wb. >>>> >>>> However, detaching an internal bdev inode from its wb in >>>> __destroy_inode() is too late. Its bdi and by extension root wb are >>>> embedded into struct request_queue, which has different lifetime rules >>>> and can be freed long before the final bdput() is called (can be from >>>> __fput() of a corresponding /dev inode, through dput() - evict() - >>>> bd_forget(). bdevs hold onto the underlying disk/queue pair only while >>>> opened; as soon as bdev is closed all bets are off. In fact, >>>> disk/queue can be gone before __blkdev_put() even returns: >>>> >>>> 1499 static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part) >>>> 1500 { >>>> ... >>>> 1518 if (bdev->bd_contains == bdev) { >>>> 1519 if (disk->fops->release) >>>> 1520 disk->fops->release(disk, mode); >>>> >>>> [ Driver puts its references to disk/queue ] >>>> >>>> 1521 } >>>> 1522 if (!bdev->bd_openers) { >>>> 1523 struct module *owner = disk->fops->owner; >>>> 1524 >>>> 1525 disk_put_part(bdev->bd_part); >>>> 1526 bdev->bd_part = NULL; >>>> 1527 bdev->bd_disk = NULL; >>>> 1528 if (bdev != bdev->bd_contains) >>>> 1529 victim = bdev->bd_contains; >>>> 1530 bdev->bd_contains = NULL; >>>> 1531 >>>> 1532 put_disk(disk); >>>> >>>> [ We put ours, the queue is gone >>>> The last bdput() would result in a write to invalid memory ] >>>> >>>> 1533 module_put(owner); >>>> ... >>>> 1539 } >>>> >>>> Since bdev inodes are special anyway, detach them in __blkdev_put() >>>> after clearing inode's dirty bits, turning the problematic >>>> inode_detach_wb() in __destroy_inode() into a noop. >>>> >>>> add_disk() grabs its disk->queue since 523e1d399ce0 ("block: make >>>> gendisk hold a reference to its queue"), so the old ->release comment >>>> is removed in favor of the new inode_detach_wb() comment. >>>> >>>> Cc: stable@vger.kernel.org # 4.2+, needs backporting >>>> Signed-off-by: Ilya Dryomov >>>> --- >>> Feel free to add >>> Tested-by: Raghavendra K T >>> >>> I was facing bad memory access problem while creating thousands of containers. >>> With this patch I am able to create more than 10k containers without hitting >>> the problem. >>> I had reported the problem here: https://lkml.org/lkml/2015/11/19/149 >> >> Great! Christoph's concern is with ->i_wb as a whole, not this >> particular patch - Al, this one is marked for stable, can we get it >> merged into -rc4? Or should it go through Jens' tree, as cgroup >> writeback patches did? > > Ping? Was holding off, but I think we should get the simple fix in for 4.4. I've applied it for this series. -- Jens Axboe