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SubjectRe: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances
On Mon, Feb 10, 2020 at 07:36:08PM -0600, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey@gmail.com> writes:
>
> > This allows to flush dcache entries of a task on multiple procfs mounts
> > per pid namespace.
> >
> > The RCU lock is used because the number of reads at the task exit time
> > is much larger than the number of procfs mounts.
>
> A couple of quick comments.
>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Signed-off-by: Djalal Harouni <tixxdz@gmail.com>
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> > ---
> > fs/proc/base.c | 20 +++++++++++++++-----
> > fs/proc/root.c | 27 ++++++++++++++++++++++++++-
> > include/linux/pid_namespace.h | 2 ++
> > include/linux/proc_fs.h | 2 ++
> > 4 files changed, 45 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 4ccb280a3e79..24b7c620ded3 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -3133,7 +3133,7 @@ static const struct inode_operations proc_tgid_base_inode_operations = {
> > .permission = proc_pid_permission,
> > };
> >
> > -static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
> > +static void proc_flush_task_mnt_root(struct dentry *mnt_root, pid_t pid, pid_t tgid)
> Perhaps just rename things like:
> > +static void proc_flush_task_root(struct dentry *root, pid_t pid, pid_t tgid)
> > {
>
> I don't think the mnt_ prefix conveys any information, and it certainly
> makes everything longer and more cumbersome.
>
> > struct dentry *dentry, *leader, *dir;
> > char buf[10 + 1];
> > @@ -3142,7 +3142,7 @@ static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
> > name.name = buf;
> > name.len = snprintf(buf, sizeof(buf), "%u", pid);
> > /* no ->d_hash() rejects on procfs */
> > - dentry = d_hash_and_lookup(mnt->mnt_root, &name);
> > + dentry = d_hash_and_lookup(mnt_root, &name);
> > if (dentry) {
> > d_invalidate(dentry);
> > dput(dentry);
> > @@ -3153,7 +3153,7 @@ static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
> >
> > name.name = buf;
> > name.len = snprintf(buf, sizeof(buf), "%u", tgid);
> > - leader = d_hash_and_lookup(mnt->mnt_root, &name);
> > + leader = d_hash_and_lookup(mnt_root, &name);
> > if (!leader)
> > goto out;
> >
> > @@ -3208,14 +3208,24 @@ void proc_flush_task(struct task_struct *task)
> > int i;
> > struct pid *pid, *tgid;
> > struct upid *upid;
> > + struct dentry *mnt_root;
> > + struct proc_fs_info *fs_info;
> >
> > pid = task_pid(task);
> > tgid = task_tgid(task);
> >
> > for (i = 0; i <= pid->level; i++) {
> > upid = &pid->numbers[i];
> > - proc_flush_task_mnt(upid->ns->proc_mnt, upid->nr,
> > - tgid->numbers[i].nr);
> > +
> > + rcu_read_lock();
> > + list_for_each_entry_rcu(fs_info, &upid->ns->proc_mounts, pidns_entry) {
> > + mnt_root = fs_info->m_super->s_root;
> > + proc_flush_task_mnt_root(mnt_root, upid->nr, tgid->numbers[i].nr);
> > + }
> > + rcu_read_unlock();
> > +
> > + mnt_root = upid->ns->proc_mnt->mnt_root;
> > + proc_flush_task_mnt_root(mnt_root, upid->nr, tgid->numbers[i].nr);
>
> I don't think this following of proc_mnt is needed. It certainly
> shouldn't be. The loop through all of the super blocks should be
> enough.

Yes, thanks!

> Once this change goes through. UML can be given it's own dedicated
> proc_mnt for the initial pid namespace, and proc_mnt can be removed
> entirely.

After you deleted the old sysctl syscall we could probably do it.

> Unless something has changed recently UML is the only other user of
> pid_ns->proc_mnt. That proc_mnt really only exists to make the loop in
> proc_flush_task easy to write.

Now I think, is there any way to get rid of proc_mounts or even
proc_flush_task somehow.

> It also probably makes sense to take the rcu_read_lock() over
> that entire for loop.

Al Viro pointed out to me that I cannot use rcu locks here :(

--
Rgrds, legion

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