linux-kernel - Re: [PATCH 2/2] nfs/blocklayout: Limit repeat device registration on failure

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Message-ID: <Zz3+rNnvxE2TRT0v@tissot.1015granger.net>
Date: Wed, 20 Nov 2024 10:22:20 -0500
From: Chuck Lever <chuck.lever@...cle.com>
To: Benjamin Coddington <bcodding@...hat.com>
Cc: Trond Myklebust <trondmy@...nel.org>, Anna Schumaker <anna@...nel.org>,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 2/2] nfs/blocklayout: Limit repeat device registration on
 failure

On Wed, Nov 20, 2024 at 09:09:35AM -0500, Benjamin Coddington wrote:
> If we're unable to register a SCSI device, ensure we mark the device as
> unavailable so that it will timeout and be re-added via GETDEVINFO.  This
> avoids repeated doomed attempts to register a device in the IO path.
> 
> Add some clarifying comments as well.
> 
> Fixes: d869da91cccb ("nfs/blocklayout: Fix premature PR key unregistration")
> Signed-off-by: Benjamin Coddington <bcodding@...hat.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> ---
>  fs/nfs/blocklayout/blocklayout.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/blocklayout/blocklayout.c b/fs/nfs/blocklayout/blocklayout.c
> index 0becdec12970..b36bc2f4f7e2 100644
> --- a/fs/nfs/blocklayout/blocklayout.c
> +++ b/fs/nfs/blocklayout/blocklayout.c
> @@ -571,19 +571,29 @@ bl_find_get_deviceid(struct nfs_server *server,
>  	if (!node)
>  		return ERR_PTR(-ENODEV);
>  
> +	/*
> +	 * Devices that are marked unavailable are left in the cache with a
> +	 * timeout to avoid sending GETDEVINFO after every LAYOUTGET, or
> +	 * constantly attempting to register the device.  Once marked as
> +	 * unavailable they must be deleted and never reused.
> +	 */
>  	if (test_bit(NFS_DEVICEID_UNAVAILABLE, &node->flags)) {
>  		unsigned long end = jiffies;
>  		unsigned long start = end - PNFS_DEVICE_RETRY_TIMEOUT;
>  
>  		if (!time_in_range(node->timestamp_unavailable, start, end)) {
> +			/* Force a new GETDEVINFO for this LAYOUT */

Or perhaps: "Uncork subsequent GETDEVINFO operations for this device"
<shrug>

>  			nfs4_delete_deviceid(node->ld, node->nfs_client, id);
>  			goto retry;
>  		}
>  		goto out_put;
>  	}
>  
> -	if (!bl_register_dev(container_of(node, struct pnfs_block_dev, node)))
> +	/* If we cannot register, treat this device as transient */

How about "Make a negative cache entry for this device"


> +	if (!bl_register_dev(container_of(node, struct pnfs_block_dev, node))) {
> +		nfs4_mark_deviceid_unavailable(node);
>  		goto out_put;
> +	}
>  
>  	return node;
>  
> -- 
> 2.47.0
> 

It took me a bit to understand what this patch does. It is like
setting up a negative dentry so the local device cache absorbs
bursts of checks for the device. OK.

Just an observation: Negative caching has some consequences too.
For instance, there will now be a period where, if the device
happens to become available, the layout is still unusable. I wonder
if that's going to have some undesirable operational effects.


-- 
Chuck Lever