Re: [PATCH] xfsprogs: ignore autofs mount table entries

[Eric Sandeen <sandeen@sandeen.net>]

On 10/7/20 8:52 PM, Ian Kent wrote:
> Some of the xfsprogs utilities read the mount table via. getmntent(3).
> 
> The mount table may contain (almost always these days since /etc/mtab is
> symlinked to /proc/self/mounts) autofs mount entries. During processing
> of the mount table entries statfs(2) can be called on mount point paths
> which will trigger an automount if those entries are direct or offset
> autofs mount triggers (indirect autofs mounts aren't affected).
> 
> This can be a problem when there are a lot of autofs direct or offset
> mounts because real mounts will be triggered when statfs(2) is called.
> This can be particularly bad if the triggered mounts are NFS mounts and
> the server is unavailable leading to lengthy boot times or worse.
> 
> Simply ignoring autofs mount entries during getmentent(3) traversals
> avoids the statfs() call that triggers these mounts. If there are
> automounted mounts (real mounts) at the time of reading the mount table
> these will still be seen in the list so they will be included if that
> actually matters to the reader.
> 
> Recent glibc getmntent(3) can ignore autofs mounts but that requires the
> autofs user to configure autofs to use the "ignore" pseudo mount option
> for autofs mounts. But this isn't yet the autofs default (to prevent
> unexpected side effects) so that can't be used.
> 
> The autofs direct and offset automount triggers are pseudo file system
> mounts and are more or less useless in terms on file system information
> so excluding them doesn't sacrifice useful file system information
> either.
> 
> Consequently excluding autofs mounts shouldn't have any adverse side
> effects.

(usually this'd go below the "---")

> Changes since v1:
> - drop hunk from fsr/xfs_fsr.c.
> 
> Signed-off-by: Ian Kent <raven@themaw.net>
> ---
>  libfrog/linux.c |    2 ++
>  libfrog/paths.c |    2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/libfrog/linux.c b/libfrog/linux.c
> index 40a839d1..a45d99ab 100644
> --- a/libfrog/linux.c
> +++ b/libfrog/linux.c
> @@ -73,6 +73,8 @@ platform_check_mount(char *name, char *block, struct stat *s, int flags)
>  	 * servers.  So first, a simple check: does the "dev" start with "/" ?
>  	 */
>  	while ((mnt = getmntent(f)) != NULL) {
> +		if (!strcmp(mnt->mnt_type, "autofs"))
> +			continue;

I may change the order of this test and the next, just so it continues to
align with the comment above.  Shouldn't make any difference, right?

Otherwise:

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

>  		if (mnt->mnt_fsname[0] != '/')
>  			continue;
>  		if (stat(mnt->mnt_dir, &mst) < 0)
> diff --git a/libfrog/paths.c b/libfrog/paths.c
> index 32737223..d6793764 100644
> --- a/libfrog/paths.c
> +++ b/libfrog/paths.c
> @@ -389,6 +389,8 @@ fs_table_initialise_mounts(
>  			return errno;
>  
>  	while ((mnt = getmntent(mtp)) != NULL) {
> +		if (!strcmp(mnt->mnt_type, "autofs"))
> +			continue;
>  		if (!realpath(mnt->mnt_dir, rmnt_dir))
>  			continue;
>  		if (!realpath(mnt->mnt_fsname, rmnt_fsname))
> 
>