linux-kernel - Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

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Message-ID: <20180312223632.GA6124@castle>
Date:   Mon, 12 Mar 2018 22:36:38 +0000
From:   Roman Gushchin <guro@...com>
To:     Al Viro <viro@...IV.linux.org.uk>
CC:     <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH 3/3] dcache: account external names as indirectly
 reclaimable memory

On Mon, Mar 12, 2018 at 09:17:42PM +0000, Al Viro wrote:
> On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 5c7df1df81ff..a0312d73f575 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -273,8 +273,16 @@ static void __d_free(struct rcu_head *head)
> >  static void __d_free_external(struct rcu_head *head)
> >  {
> >  	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
> > -	kfree(external_name(dentry));
> > -	kmem_cache_free(dentry_cache, dentry); 
> > +	struct external_name *name = external_name(dentry);
> > +	unsigned long bytes;
> > +
> > +	bytes = dentry->d_name.len + offsetof(struct external_name, name[1]);
> > +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> > +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> > +			    -kmalloc_size(kmalloc_index(bytes)));
> > +
> > +	kfree(name);
> > +	kmem_cache_free(dentry_cache, dentry);
> >  }
> 
> That can't be right - external names can be freed in release_dentry_name_snapshot()
> and copy_name() as well.  When do you want kfree_rcu() paths accounted for, BTW?
> At the point where we are freeing them, or where we are scheduling their freeing?

Ah, I see...

I think, it's better to account them when we're actually freeing,
otherwise we will have strange path:
(indirectly) reclaimable -> unreclaimable -> free

Do you agree?

Although it shouldn't be that important in practice.

Thank you!

--

>From ad9d6c627c2b9315de1967c40a1f4fa68705cf9e Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@...com>
Date: Mon, 12 Mar 2018 22:24:28 +0000
Subject: [PATCH] dcache: fix indirectly reclaimable memory accounting

Signed-off-by: Roman Gushchin <guro@...com>
---
 fs/dcache.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 98826efe22a0..19bc7495a6c4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -266,6 +266,19 @@ static void __d_free(struct rcu_head *head)
 	kmem_cache_free(dentry_cache, dentry); 
 }
 
+static void __d_free_external_name(struct rcu_head *head)
+{
+	struct external_name *name;
+
+	name = container_of(head, struct external_name, u.head);
+
+	mod_node_page_state(page_pgdat(virt_to_page(name)),
+			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
+			    -ksize(name));
+
+	kfree(name);
+}
+
 static void __d_free_external(struct rcu_head *head)
 {
 	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
@@ -307,7 +320,7 @@ void release_dentry_name_snapshot(struct name_snapshot *name)
 		struct external_name *p;
 		p = container_of(name->name, struct external_name, name[0]);
 		if (unlikely(atomic_dec_and_test(&p->u.count)))
-			kfree_rcu(p, u.head);
+			call_rcu(&p->u.head, __d_free_external_name);
 	}
 }
 EXPORT_SYMBOL(release_dentry_name_snapshot);
@@ -2769,7 +2782,7 @@ static void copy_name(struct dentry *dentry, struct dentry *target)
 		dentry->d_name.hash_len = target->d_name.hash_len;
 	}
 	if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
-		kfree_rcu(old_name, u.head);
+		call_rcu(&old_name->u.head, __d_free_external_name);
 }
 
 static void dentry_lock_for_move(struct dentry *dentry, struct dentry *target)
-- 
2.14.3