Re: [PATCH] RFC: autofs-5.1.9 - flag removed entries as stale

On 1/8/25 23:22, David Disseldorp wrote:
> This effectively reverts commit 21ce28d ("autofs-5.1.4 - mark removed
> cache entry negative"), which causes the kernel to stall in autofs_wait
> for the following workload:
>
>    cat > /etc/auto.direct <<EOF
>    echo "/nfs/share  $mount_args ${NFS_SERVER}:/${NFS_SHARE}"
>    EOF
>
>    setsid --fork automount --debug --foreground &> /automount.log
>    sleep 1
>
>    touch /test.run
>    setsid --fork /bin/bash -c \
>      "while [[ -f /test.run ]]; do df -ia >> /test.log; sleep 1; done"
>    echo "df loop logging to /test.log"
>
>    sleep 2
>    echo "changing and reloading auto.direct"
>    echo > /etc/auto.direct
>    killall -HUP automount
>
>    sleep 2
>    echo "unmounting..."
>    umount /nfs/share || echo "umount failed"
>
> The current behaviour sees us hit:
>    handle_packet_missing_direct:1352: can't find map entry for ()
> ...which doesn't respond to the kernel, triggering the stall.
>
> This approach adds a new MOUNT_FLAG_STALE flag to track removed map
> entries. While keeping enough state around to respond for the
> handle_packet_missing_direct case.
>
> RFC:
> - needs further testing (e.g. indirect maps)
> - I'm not familiar with the codebase so this may be the wrong approach
> - we may need a background job to purge MOUNT_FLAG_STALE entries?

In the light of the work that I did reviewing the map entry cache pruning

this patch seems not quite right but it is a good approach and I do like

it. I'll see if I can work it into what I've done to the cache pruning

without breaking it.


Ian

>
> Signed-off-by: David Disseldorp <[email protected]>
> ---
>   daemon/direct.c     |  8 ++++++--
>   daemon/indirect.c   |  8 ++++++--
>   daemon/lookup.c     | 11 ++++-------
>   include/automount.h |  3 +++
>   4 files changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/daemon/direct.c b/daemon/direct.c
> index 42baac8..5e78c40 100644
> --- a/daemon/direct.c
> +++ b/daemon/direct.c
> @@ -1389,8 +1389,12 @@ int handle_packet_missing_direct(struct autofs_point *ap, autofs_packet_missing_
>   		return 0;
>   	}
>   
> -	/* Check if we recorded a mount fail for this key */
> -	if (me->status >= monotonic_time(NULL)) {
> +	/*
> +	 * Check if we recorded a mount fail for this key, or the entry has
> +	 * been removed.
> +	 */
> +	if (me->status >= monotonic_time(NULL) ||
> +	    me->flags & MOUNT_FLAG_STALE) {
>   		ops->send_fail(ap->logopt,
>   			       ioctlfd, pkt->wait_queue_token, -ENOENT);
>   		ops->close(ap->logopt, ioctlfd);
> diff --git a/daemon/indirect.c b/daemon/indirect.c
> index 7d4aad7..934bb74 100644
> --- a/daemon/indirect.c
> +++ b/daemon/indirect.c
> @@ -798,8 +798,12 @@ int handle_packet_missing_indirect(struct autofs_point *ap, autofs_packet_missin
>   
>   	me = lookup_source_mapent(ap, pkt->name, LKP_DISTINCT);
>   	if (me) {
> -		/* Check if we recorded a mount fail for this key */
> -		if (me->status >= monotonic_time(NULL)) {
> +		/*
> +		 * Check if we recorded a mount fail for this key, or the entry
> +		 * has been removed.
> +		 */
> +		if (me->status >= monotonic_time(NULL) ||
> +		    me->flags & MOUNT_FLAG_STALE) {
>   			ops->send_fail(ap->logopt, ap->ioctlfd,
>   				       pkt->wait_queue_token, -ENOENT);
>   			cache_unlock(me->mc);
> diff --git a/daemon/lookup.c b/daemon/lookup.c
> index dc77948..ad0b460 100644
> --- a/daemon/lookup.c
> +++ b/daemon/lookup.c
> @@ -1416,15 +1416,12 @@ void lookup_prune_one_cache(struct autofs_point *ap, struct mapent_cache *mc, ti
>   		if (valid && valid->mc == mc) {
>   			 /*
>   			  * We've found a map entry that has been removed from
> -			  * the current cache so it isn't really valid. Set the
> -			  * mapent negative to prevent further mount requests
> +			  * the current cache so it isn't really valid. Flag the
> +			  * mapent stale to prevent further mount requests
>   			  * using the cache entry.
>   			  */
> -			debug(ap->logopt, "removed map entry detected, mark negative");
> -			if (valid->mapent) {
> -				free(valid->mapent);
> -				valid->mapent = NULL;
> -			}
> +			debug(ap->logopt, "removed map entry detected, mark stale");
> +			valid->flags |= MOUNT_FLAG_STALE;
>   			cache_unlock(valid->mc);
>   			valid = NULL;
>   		}
> diff --git a/include/automount.h b/include/automount.h
> index 9548db8..007d020 100644
> --- a/include/automount.h
> +++ b/include/automount.h
> @@ -548,6 +548,9 @@ struct kernel_mod_version {
>   /* Indicator for applications to ignore the mount entry */
>   #define MOUNT_FLAG_IGNORE		0x1000
>   
> +/* map has been removed, but we can't clean up yet */
> +#define MOUNT_FLAG_STALE		0x2000
> +
>   struct autofs_point {
>   	pthread_t thid;
>   	char *path;			/* Mount point name */