From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 18701C3B189 for ; Thu, 13 Feb 2020 08:27:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CBA2721734 for ; Thu, 13 Feb 2020 08:27:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="DhB5E3Vv" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726232AbgBMI1C (ORCPT ); Thu, 13 Feb 2020 03:27:02 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:21207 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729440AbgBMI1C (ORCPT ); Thu, 13 Feb 2020 03:27:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1581582420; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=z12raJGzmP3ewFFkzOjLyM0T/GP1BeNJQ4XTGkx2X3s=; b=DhB5E3VvAhQ/n9aoeNmupbvFtzMTRTy67FVDjjfXYBFV3pE2+7ACiG0LAmKe6d83itL0Qc Ni75ZplAfVy5dLCbOA3fsO97ZwQDHfCjaD1ryzhO13EdyTC8dgCdOOhxqqev1/0QSyPlDT q2WYQBCfvaIgl4C8zfFbBDkx5fQALu0= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-147-eGc5_gbMMi6FGS17bB_EvA-1; Thu, 13 Feb 2020 03:26:56 -0500 X-MC-Unique: eGc5_gbMMi6FGS17bB_EvA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 01D0F1088383; Thu, 13 Feb 2020 08:26:55 +0000 (UTC) Received: from ming.t460p (ovpn-8-17.pek2.redhat.com [10.72.8.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 158E28EA1A; Thu, 13 Feb 2020 08:26:47 +0000 (UTC) Date: Thu, 13 Feb 2020 16:26:43 +0800 From: Ming Lei To: Salman Qazi Cc: Jens Axboe , Bart Van Assche , Christoph Hellwig , Linux Kernel Mailing List , linux-block@vger.kernel.org, Gwendal Grignou , Jesse Barnes Subject: Re: BLKSECDISCARD ioctl and hung tasks Message-ID: <20200213082643.GB9144@ming.t460p> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Feb 12, 2020 at 02:27:09PM -0800, Salman Qazi wrote: > Hi, > > So, here's another issue that we are grappling with, where we have a > root-cause but don't currently have a good fix for. BLKSECDISCARD is > an operation used for securely destroying a subset of the data on a > device. Unfortunately, on SSDs, this is an operation with variable > performance. It can be O(minutes) in the worst case. The > pathological case is when many erase blocks on the flash contain a > small amount of data that is part of the discard and a large amount of > data that isn't. In such cases, the erase blocks have to be copied > almost in entirety to fresh blocks, in order to erase the sectors to > be discarded. This can be thought of as a defragmentation operation on > the drive and can be expected to cost in the same ballpark as > rewriting most of the contents of the drive. > > Therefore, it is possible for the thread waiting in the IOCTL in > submit_bio_wait call in blkdev_issue_discard to wait for several > minutes. The hung task watchdog is usually configured for 2 minutes, > and this can expire before the operation finishes. > > This operation is very important to the security model of Chrome OS > devices. Therefore, we would like the kernel to survive this even if > it takes several minutes. > > Three approaches come to mind: > > One approach is to somehow avoid waiting for a single monolithic > operation and instead wait on bits and pieces of the operation. These > can be sized to finish within a reasonable timeframe. The exact size > is likely device-specific. We already split these operations before > issuing to the device, but the IOCTL thread is waiting for the whole > rather than the parts. The hung task watchdog only sees the total > amount of time the thread slept and not the forward progress taking > place quietly. > > Another approach might be to do something in the spirit of the write > system call: complete the partial operation (whatever the kernel > thinks is reasonable), adjust the IOCTL argument and have the > userspace reissue the syscall to continue the operation. The second > option should probably be done with a different IOCTL name to avoid > breaking userspace. > > A third approach, which is perhaps more adventurous, is to have a > notion of forward progress that a thread can export and the hung task > watchdog can evaluate. This can take the form of a function pointer > and an argument. The result of the function is a monotonically > decreasing unsigned value. When this value stops changing, we can > conclude that the thread is hung. This can be used in place of > context switch count for tasks where this function is available. This > can potentially solve other similar issues, there is a way to tell if > there is forward progress, but it is not as straightforward as the > context switch count. > > What are your thoughts? The approach used in blk_execute_rq() can be borrowed for workaround the issue, such as: diff --git a/block/bio.c b/block/bio.c index 94d697217887..c9ce19a86de7 100644 --- a/block/bio.c +++ b/block/bio.c @@ -17,6 +17,7 @@ #include #include #include +#include #include #include "blk.h" @@ -1019,12 +1020,19 @@ static void submit_bio_wait_endio(struct bio *bio) int submit_bio_wait(struct bio *bio) { DECLARE_COMPLETION_ONSTACK_MAP(done, bio->bi_disk->lockdep_map); + unsigned long hang_check; bio->bi_private = &done; bio->bi_end_io = submit_bio_wait_endio; bio->bi_opf |= REQ_SYNC; submit_bio(bio); - wait_for_completion_io(&done); + + /* Prevent hang_check timer from firing at us during very long I/O */ + hang_check = sysctl_hung_task_timeout_secs; + if (hang_check) + while (!wait_for_completion_io_timeout(&done, hang_check * (HZ/2))); + else + wait_for_completion_io(&done); return blk_status_to_errno(bio->bi_status); } thanks, Ming