Re: [PATCH 5/6] XFS: remove congestion_wait() loop from kmem_alloc()

[Dave Chinner]

On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote:
> Documentation commment in gfp.h discourages indefinite retry loops on
> ENOMEM and says of __GFP_NOFAIL that it
> 
>     is definitely preferable to use the flag rather than opencode
>     endless loop around allocator.
> 
> So remove the loop, instead specifying __GFP_NOFAIL if KM_MAYFAIL was
> not given.
> 
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
>  fs/xfs/kmem.c |   16 ++++------------
>  1 file changed, 4 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/xfs/kmem.c b/fs/xfs/kmem.c
> index 6f49bf39183c..f545f3633f88 100644
> --- a/fs/xfs/kmem.c
> +++ b/fs/xfs/kmem.c
> @@ -13,19 +13,11 @@ kmem_alloc(size_t size, xfs_km_flags_t flags)
>  {
>  	int	retries = 0;
>  	gfp_t	lflags = kmem_flags_convert(flags);
> -	void	*ptr;
>  
>  	trace_kmem_alloc(size, flags, _RET_IP_);
>  
> -	do {
> -		ptr = kmalloc(size, lflags);
> -		if (ptr || (flags & KM_MAYFAIL))
> -			return ptr;
> -		if (!(++retries % 100))
> -			xfs_err(NULL,
> -	"%s(%u) possible memory allocation deadlock size %u in %s (mode:0x%x)",
> -				current->comm, current->pid,
> -				(unsigned int)size, __func__, lflags);
> -		congestion_wait(BLK_RW_ASYNC, HZ/50);
> -	} while (1);
> +	if (!(flags & KM_MAYFAIL))
> +		lflags |= __GFP_NOFAIL;
> +
> +	return kmalloc(size, lflags);
>  }

Which means we no longer get warnings about memory allocation
failing - kmem_flags_convert() sets __GFP_NOWARN for all allocations
in this loop. Hence we'll now get silent deadlocks through this code
instead of getting warnings that memory allocation is failing
repeatedly.

I also wonder about changing the backoff behaviour here (it's a 20ms
wait right now because there are not early wakeups) will affect the
behaviour, as __GFP_NOFAIL won't wait for that extra time between
allocation attempts....

And, of course, how did you test this? Sometimes we see
unpredicted behaviours as a result of "simple" changes like this
under low memory conditions...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx