Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

[Alexey Gladkov <gladkov.alexey@gmail.com>]

On Wed, Feb 12, 2020 at 08:59:29AM -0600, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey@gmail.com> writes:
> 
> > 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 :(
> 
> Fundamentally proc_flush_task is an optimization.  Just getting rid of
> dentries earlier.  At least at one point it was an important
> optimization because the old process dentries would just sit around
> doing nothing for anyone.
> 
> I wonder if instead of invalidating specific dentries we could instead
> fire wake up a shrinker and point it at one or more instances of proc.
> 
> The practical challenge I see is something might need to access the
> dentries to see that they are invalid.
> 
> We definitely could try without this optimization and see what happens.

When Linus said that a semaphore for proc_mounts is a bad idea, I tried
to come up with some kind of asynchronous way to clear it per superblock.
I gave up with the asynchronous GC because userspace can quite easily get
ahead of it.

Without this optimization the kernel starts to consume a lot of memory
during intensive reading /proc. I tried to do:

while :; do
	for x in `seq 0 9`; do sleep 0.1; done;
	ls /proc/[0-9]*;
done >/dev/null;

and memory consumption went up without proc_flush_task. Since we have
mounted procfs in each container, this is dangerous.

-- 
Rgrds, legion