Re: Silent data corruption in blkdev_direct_IO()
From: Martin Wilck <hidden>
Date: 2018-07-13 22:38:16
On Fri, 2018-07-13 at 12:50 -0600, Jens Axboe wrote:
On 7/13/18 12:00 PM, Jens Axboe wrote:quoted
On 7/13/18 10:56 AM, Martin Wilck wrote:quoted
On Thu, 2018-07-12 at 10:42 -0600, Jens Axboe wrote:quoted
Hence the patch I sent is wrong, the code actually looks fine. Which means we're back to trying to figure out what is going on here. It'd be great with a test case...We don't have an easy test case yet. But the customer has confirmed that the problem occurs with upstream 4.17.5, too. We also confirmed again that the problem occurs when the kernel uses the kmalloc() code path in __blkdev_direct_IO_simple(). My personal suggestion would be to ditch __blkdev_direct_IO_simple() altogether. After all, it's not _that_ much simpler thatn __blkdev_direct_IO(), and it seems to be broken in a subtle way.That's not a great suggestion at all, we need to find out why we're hitting the issue. For all you know, the bug could be elsewhere and we're just going to be hitting it differently some other way. The head-in-the-sand approach is rarely a win long term. It's saving an allocation per IO, that's definitely measurable on the faster storage. For reads, it's also not causing a context switch for dirtying pages. I'm not a huge fan of multiple cases in general, but this one is definitely warranted in an era where 1 usec is a lot of extra time for an IO.quoted
However, so far I've only identified a minor problem, see below - it doesn't explain the data corruption we're seeing.What would help is trying to boil down a test case. So far it's a lot of hand waving, and nothing that can really help narrow down what is going on here.When someone reports to you that some kind of data corruption occurs, you need to find out as many details you can. Ideally they can give you a test case, but if they can't, you ask as many questions as possible to help YOU make a test case. - What is the nature of the corruption? Is it reads or writes? Is it zeroes, random data, or data from somewhere else? How big is the corrupted area? - What does the workload look like that reproduces it? If they don't really know and they can't give you a reproducer, you help them with tracing to give you the information you need. Those are a great start. Right now we have zero information, other than reverting that patch fixes it. This means that we're likely dealing with IO that is larger than 16k, since we're going to be hitting the same path for <= 16k ios with the patch reverted. We also know it's less than 1MB.
The nature of the corruption is an interesting question. The weird thing is that it's not block-aligned - "bad" data starts at some apparently random offset into the log file. That'd actually made me think that we're not looking at a block IO issue but maybe some corruption that occured in memory before write-out. But then we isolated this specific patch by bisection... I had my doubts, but it seems to be sound, at least the test results indicate so. The user application is a database which writes logs in blocks that are multiples of 8k. The IO goes through various layers in the VM (ext3 on LVM on MD on virtio), then through qemu via virtio-scsi, and onto a host multipath device which is backed by FC storage. The VM is running an old SLES 11 distro, the problems were caused by a host kernel update. My attempts to reproduce a corruption with lab equipment have failed up to now. I've been trying to replay the customer's workload with btreplay. I also tried to replay it with fio with verification to try to find corruptions, but I guess I didn't get the setup quite right.
But that's it. I don't even know if it's writes, since your (and Hannes's) report has no details at all. Go look at what you ask customers to provide for bug reports, then apply some of that to this case...
Sorry. It's writes. The reason we posted this here was a) the concrete questions Hannes asked about the code, b) the patch we'd isolated - we thought you or Christoph might just have a bright idea about it, or see what we didn't, and c) that we thought the problem might affect upstream, too. We don't expect you to do the dirty work for us. We'll come back to the list when we know more, or have specific questions. Martin -- Dr. Martin Wilck [off-list ref], Tel. +49 (0)911 74053 2107 SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg)