Re: [PATCH] xfs: fall back to vmalloc when allocation log vector buffers

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Feb 01, 2018 at 12:39:59PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> When using large directory blocks, we regularly see memory
> allocations of >64k being made for the shadow log vector buffer.
> When we are under memory pressure, kmalloc() may not be able to find
> contiguous memory chunks large enough to satisfy these allocations
> easily, and if memory is fragmented we can potentially stall here.
> 
> TO avoid this problem, switch the log vector buffer allocation to
> use kmem_alloc_large(). This will allow failed allocations to fall
> back to vmalloc and so remove the dependency on large contiguous
> regions of memory being available. This should prevent slowdowns
> and potential stalls when memory is low and/or fragmented.

> Signed-Off-By: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/kmem.c        | 6 +++---
>  fs/xfs/kmem.h        | 8 +++++++-
>  fs/xfs/xfs_log_cil.c | 2 +-
>  3 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/kmem.c b/fs/xfs/kmem.c
> index 393b6849aeb3..7bace03dc9dc 100644
> --- a/fs/xfs/kmem.c
> +++ b/fs/xfs/kmem.c
> @@ -46,13 +46,13 @@ kmem_alloc(size_t size, xfs_km_flags_t flags)
>  }
>  
>  void *
> -kmem_zalloc_large(size_t size, xfs_km_flags_t flags)
> +kmem_alloc_large(size_t size, xfs_km_flags_t flags)
>  {
>  	unsigned nofs_flag = 0;
>  	void	*ptr;
>  	gfp_t	lflags;
>  
> -	ptr = kmem_zalloc(size, flags | KM_MAYFAIL);
> +	ptr = kmem_alloc(size, flags | KM_MAYFAIL);
>  	if (ptr)
>  		return ptr;
>  
> @@ -67,7 +67,7 @@ kmem_zalloc_large(size_t size, xfs_km_flags_t flags)
>  		nofs_flag = memalloc_nofs_save();
>  
>  	lflags = kmem_flags_convert(flags);
> -	ptr = __vmalloc(size, lflags | __GFP_ZERO, PAGE_KERNEL);
> +	ptr = __vmalloc(size, lflags, PAGE_KERNEL);
>  
>  	if (flags & KM_NOFS)
>  		memalloc_nofs_restore(nofs_flag);
> diff --git a/fs/xfs/kmem.h b/fs/xfs/kmem.h
> index 4b87472f35bc..6023b594ead7 100644
> --- a/fs/xfs/kmem.h
> +++ b/fs/xfs/kmem.h
> @@ -71,7 +71,7 @@ kmem_flags_convert(xfs_km_flags_t flags)
>  }
>  
>  extern void *kmem_alloc(size_t, xfs_km_flags_t);
> -extern void *kmem_zalloc_large(size_t size, xfs_km_flags_t);
> +extern void *kmem_alloc_large(size_t size, xfs_km_flags_t);
>  extern void *kmem_realloc(const void *, size_t, xfs_km_flags_t);
>  static inline void  kmem_free(const void *ptr)
>  {
> @@ -85,6 +85,12 @@ kmem_zalloc(size_t size, xfs_km_flags_t flags)
>  	return kmem_alloc(size, flags | KM_ZERO);
>  }
>  
> +static inline void *
> +kmem_zalloc_large(size_t size, xfs_km_flags_t flags)
> +{
> +	return kmem_alloc_large(size, flags | KM_ZERO);
> +}
> +
>  /*
>   * Zone interfaces
>   */
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 43aa42a3a5d3..61ab5c0a4c45 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -202,7 +202,7 @@ xlog_cil_alloc_shadow_bufs(
>  			 */
>  			kmem_free(lip->li_lv_shadow);
>  
> -			lv = kmem_alloc(buf_size, KM_SLEEP|KM_NOFS);
> +			lv = kmem_alloc_large(buf_size, KM_SLEEP|KM_NOFS);

"KM_SLEEP | KM_NOFS", will fix it on the way in.

I wonder about expanding the uses of vmalloc space, but otoh stalling
the log seems like a worse idea...

Looks ok enough to give it a spin,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

>  			memset(lv, 0, xlog_cil_iovec_space(niovecs));
>  
>  			lv->lv_item = lip;
> -- 
> 2.15.1
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html