linux-kernel - Re: [PATCH v4 10/12] dax: add struct iomap based DAX PMD support

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Date:   Thu, 6 Oct 2016 20:58:33 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH v4 10/12] dax: add struct iomap based DAX PMD support

On Thu, Oct 06, 2016 at 03:34:24PM -0600, Ross Zwisler wrote:
> Interesting - adding iomap_end() calls to the DAX PTE fault handler causes an
> AA deadlock because we try and retake ei->dax_sem.  We take dax_sem in
> ext2_dax_fault() before calling into the DAX code, then if we end up going
> through the error path in ext2_iomap_end(), we call 
>   ext2_write_failed()
>     ext2_truncate_blocks()
>       dax_sem_down_write()
> 
> Where we try and take dax_sem again.  This error path is really only valid for
> I/O operations, but we happen to call it for page faults because 'written' in
> ext2_iomap_end() is just 0.
> 
> So...how should we handle this?  A few ideas:
> 
> 1) Just continue to omit the calls to iomap_end() in the DAX page fault
> handlers for now, and add them when there is useful work to be done in one of
> the filesystems.
> 
> 2) Add an IOMAP_FAULT flag to the flags passed into iomap_begin() and
> iomap_end() so make it explicit that we are calling as part of a fault handler
> and not an I/O operation, and use this to adjust the error handling in
> ext2_iomap_end().
> 
> 3) Just work around the existing error handling in ext2_iomap_end() by either
> unsetting IOMAP_WRITE or by setting 'written' to the size of the fault.
> 
> For #2 or #3, probably add a comment explaining the deadlock and why we need
> to never call ext2_write_failed() while handling a page fault.
> 
> Thoughts?

Never mind, #3 it is, I think it was just a plain bug to call iomap_end() with
'length' != 'written'.