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Subject: Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        Anju T Sudhakar <anju@linux.vnet.ibm.com>,
        darrick.wong@oracle.com, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        willy@infradead.org
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 <20200821044533.BBFD1A405F@d06av23.portsmouth.uk.ibm.com>
 <20200821060025.GA31091@infradead.org>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Fri, 21 Aug 2020 14:39:33 +0530
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On 8/21/20 11:30 AM, Christoph Hellwig wrote:
> On Fri, Aug 21, 2020 at 10:15:33AM +0530, Ritesh Harjani wrote:
>> Please correct me here, but as I see, bio has only these two limits
>> which it checks for adding page to bio. It doesn't check for limits
>> of /sys/block/<dev>/queue/* no? I guess then it could be checked
>> by block layer below b4 submitting the bio?
> 
> The bio does not, but the blk-mq code will split the bios when mapping
> it to requests, take a look at blk_mq_submit_bio and __blk_queue_split.

Thanks :)

> 
> But while the default limits are quite low, they can be increased
> siginificantly, which tends to help with performance and is often
> also done by scripts shipped by the distributions.
> 
>> This issue was first observed while running a fio run on a system with
>> huge memory. But then here is an easy way we figured out to trigger the
>> issue almost everytime with loop device on my VM setup. I have provided
>> all the details on this below.
> 
> Can you wire this up for xfstests?
> 

Sure, will do that.
I do see generic/460 does play with such vm dirty_ratio params which
this test would also require to manipulate to trigger this issue.


Thanks for the suggestion!
-ritesh