Re: mm allocation failure and hang when running xfstests generic/269 on xfs

[Brian Foster <bfoster@redhat.com>]

On Thu, Mar 02, 2017 at 11:35:20AM +0100, Michal Hocko wrote:
> On Thu 02-03-17 19:04:48, Tetsuo Handa wrote:
> [...]
> > So, commit 5d17a73a2ebeb8d1("vmalloc: back off when the current task is
> > killed") implemented __GFP_KILLABLE flag and automatically applied that
> > flag. As a result, those who are not ready to fail upon SIGKILL are
> > confused. ;-)
> 
> You are right! The function is documented it might fail but the code
> doesn't really allow that. This seems like a bug to me. What do you
> think about the following?
> ---
> From d02cb0285d8ce3344fd64dc7e2912e9a04bef80d Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Thu, 2 Mar 2017 11:31:11 +0100
> Subject: [PATCH] xfs: allow kmem_zalloc_greedy to fail
> 
> Even though kmem_zalloc_greedy is documented it might fail the current
> code doesn't really implement this properly and loops on the smallest
> allowed size for ever. This is a problem because vzalloc might fail
> permanently. Since 5d17a73a2ebe ("vmalloc: back off when the current
> task is killed") such a failure is much more probable than it used to
> be. Fix this by bailing out if the minimum size request failed.
> 
> This has been noticed by a hung generic/269 xfstest by Xiong Zhou.
> 
> Reported-by: Xiong Zhou <xzhou@redhat.com>
> Analyzed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  fs/xfs/kmem.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/xfs/kmem.c b/fs/xfs/kmem.c
> index 339c696bbc01..ee95f5c6db45 100644
> --- a/fs/xfs/kmem.c
> +++ b/fs/xfs/kmem.c
> @@ -34,6 +34,8 @@ kmem_zalloc_greedy(size_t *size, size_t minsize, size_t maxsize)
>  	size_t		kmsize = maxsize;
>  
>  	while (!(ptr = vzalloc(kmsize))) {
> +		if (kmsize == minsize)
> +			break;
>  		if ((kmsize >>= 1) <= minsize)
>  			kmsize = minsize;
>  	}

More consistent with the rest of the kmem code might be to accept a
flags argument and do something like this based on KM_MAYFAIL. The one
current caller looks like it would pass it, but I suppose we'd still
need a mechanism to break out should a new caller not pass that flag.
Would a fatal_signal_pending() check in the loop as well allow us to
break out in the scenario that is reproduced here?

Brian

> -- 
> 2.11.0
> 
> -- 
> Michal Hocko
> SUSE Labs
> 
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