Re: xfs_extent_busy_flush vs. aio

[Brian Foster <bfoster@redhat.com>]

On Tue, Jan 23, 2018 at 04:57:03PM +0200, Avi Kivity wrote:
> I'm seeing the equivalent[*] of xfs_extent_busy_flush() sleeping in my
> beautiful io_submit() calls.
> 
> 
> Questions:
> 
>  - Is it correct that RWF_NOWAIT will not detect the condition that led to
> the log being forced?
> 
>  - If so, can it be fixed?
> 
>  - Can I do something to reduce the odds of this occurring? larger logs,
> more logs, flush more often, resurrect extinct species and sacrifice them to
> the xfs gods?
> 
>  - Can an xfs developer do something? For example, make it RWF_NOWAIT
> friendly (if the answer to the first question was "correct")
> 

So RWF_NOWAIT eventually works its way to IOMAP_NOWAIT, which looks like
it skips any write call that would require allocation in
xfs_file_iomap_begin(). The busy flush should only happen in the block
allocation path, so something is missing here. Do you have a backtrace
for the log force you're seeing?

Brian

> 
> [*] equivalent, because I'm actually looking at an older kernel that lacks
> this function. But I'm moderately confident that the xfs_log_force I'm
> seeing was transformed into xfs_extent_busy_flush by
> ebf55872616c7d4754db5a318591a72a8d5e6896
> 
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