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

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

On Thu, Oct 01, 2020 at 09:06:31AM +0800, 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.
> 
> Signed-off-by: Ian Kent <raven@themaw.net>
> ---
>  fsr/xfs_fsr.c   |    3 +++
>  libfrog/linux.c |    2 ++
>  libfrog/paths.c |    2 ++
>  3 files changed, 7 insertions(+)
> 
> diff --git a/fsr/xfs_fsr.c b/fsr/xfs_fsr.c
> index 77a10a1d..466ad9e4 100644
> --- a/fsr/xfs_fsr.c
> +++ b/fsr/xfs_fsr.c
> @@ -323,6 +323,9 @@ initallfs(char *mtab)
>  	while ((mnt = platform_mntent_next(&cursor)) != NULL) {
>  		int rw = 0;
>  
> +		if (!strcmp(mnt->mnt_type, "autofs"))
> +			continue;

Hmm...  the libfrog changes look decent, but it strikes me as a little
odd that we don't just make platform_mntent_next filter that out?

(Or I guess refactor fsr to use the fs table...)

OTOH "Not _another_ herring^Wrefactor!"

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> +
>  		if (strcmp(mnt->mnt_type, MNTTYPE_XFS ) != 0 ||
>  		    stat(mnt->mnt_fsname, &sb) == -1 ||
>  		    !S_ISBLK(sb.st_mode))
> 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;
>  		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))
> 
>