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SubjectRe: [PATCH 3/4] vfs: check for autofs expiring dentry in follow_automount()
Date
On Thu, Mar 26, 2020 at 01:23:29PM +0800, Ian Kent wrote:
>follow_automount() checks if a stat family system call path walk is
>being done on a positive dentry and and returns -EISDIR to indicate
>the dentry should be used as is without attempting an automount.
>
>But if autofs is expiring the dentry at the time it should be
>remounted following the expire.
>
>There are two cases, in the case of a "nobrowse" indirect autofs
>mount it would have been mounted on lookup anyway. In the case of
>a "browse" indirect or direct autofs mount re-mounting it will
>maintain the mount which is what user space would be expected.
>
>This will defer expiration of the mount which might lead to mounts
>unexpectedly remaining mounted under heavy stat activity but there's
>no other choice in order to maintain consistency for user space.
>
>Signed-off-by: Ian Kent <raven@themaw.net>
>---
> fs/autofs/root.c | 10 +++++++++-
> fs/namei.c | 13 +++++++++++--
> 2 files changed, 20 insertions(+), 3 deletions(-)
>
>diff --git a/fs/autofs/root.c b/fs/autofs/root.c
>index a1c9c32e104f..308cc49aca1d 100644
>--- a/fs/autofs/root.c
>+++ b/fs/autofs/root.c
>@@ -406,9 +406,17 @@ static int autofs_d_manage(const struct path *path, bool rcu_walk)
>
> /* Check for (possible) pending expire */
> if (ino->flags & AUTOFS_INF_WANT_EXPIRE) {
>+ /* dentry possibly going to be picked for expire,
>+ * proceed to ref-walk mode.
>+ */
> if (rcu_walk)
> return -ECHILD;
>- return 0;
>+
>+ /* ref-walk mode, return 1 so follow_automount()
>+ * can to wait on the expire outcome and possibly

'can to wait' ?
Do you mean: "can wait", "will wait", "knows to wait",
or something else?

>+ * attempt a re-mount.
>+ */
>+ return 1;
> }
>
> /*
>diff --git a/fs/namei.c b/fs/namei.c
>index db6565c99825..34a03928d32d 100644
>--- a/fs/namei.c
>+++ b/fs/namei.c
>@@ -1227,11 +1227,20 @@ static int follow_automount(struct path *path, struct nameidata *nd,
> * mounted directory. Also, autofs may mark negative dentries
> * as being automount points. These will need the attentions
> * of the daemon to instantiate them before they can be used.
>+ *
>+ * Also if ->d_manage() returns 1 the dentry transit needs
>+ * managing, for autofs it tells us the dentry might be expired,
>+ * so proceed to ->d_automount().

I'm wondering if this should be two sentences.
Does this version reflect reality?

+ * Also if ->d_manage() returns 1 the dentry transit needs
+ * to be managed. For autofs, a return of 1 tells us the
+ * dentry might be expired, so proceed to ->d_automount().

Kind regards
Vince
> */
> if (!(nd->flags & (LOOKUP_PARENT | LOOKUP_DIRECTORY |
> LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_AUTOMOUNT)) &&
>- path->dentry->d_inode)
>- return -EISDIR;
>+ path->dentry->d_inode) {
>+ if (path->dentry->d_flags & DCACHE_MANAGE_TRANSIT) {
>+ if (!path->dentry->d_op->d_manage(path, false))
>+ return -EISDIR;
>+ } else
>+ return -EISDIR;
>+ }
>
> nd->total_link_count++;
> if (nd->total_link_count >= 40)
>

--
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