[Devel] [PATCH RHEL8 COMMIT] kernel/ve: simplify ve_get_init_css

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-240.1.1.vz8.5.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-240.1.1.vz8.5.59
------>
commit 2388651c365bb9fde54885e87e5454cfaa90d716
Author: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Date:   Fri Jul 9 02:23:10 2021 +0300

    kernel/ve: simplify ve_get_init_css
    
    Reuse css_local_root to ascend to marked css.
    
    https://jira.sw.ru/browse/PSBM-131253
    
    Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
    Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>
    
    ktkhai@: Looks OK. Since child is definitely alive (BUG_ON() even says
    this), parent is also alive. So, we may simplify this.
    
    v2:
    khorenko@: use rcu_dereference() for ve->ve_ns access.
    At least this explains why do we use rcu_read_[un]lock() here.
    
    ==========================
    mm/writeback: Adopt cgroup-v2 writeback (limit per-memcg dirty memory)
    
    In cgroup-v1 all writeback IO is accounted to root blkcg by design. With
    cgroup-v2 it became possible to link memcg and blkcg, so writeback code
    was enhanced to
     1) consider balancing dirty pages per memory cgroup
     2) account writeback generated IO to blkcg
    
    In vz7 writeback was balancing by means of beancounter cgroup. However we
    dropped it.
    
    In vz8 @aryabinin tried to enable cgroup-v2 writeback with 5cc286c98ee20
    ("mm, cgroup, writeback: Enable per-cgroup writeback for v1 cgroup."),
    but cgroup_get_e_css(), which is used to find blkcg based on memcg,
    does not work well with cgroup-v1 and always returns root blkcg.
    
    However we can implement a new function to associate blkcg with memcg via
    ve css_set.
    
    Test results with 256M container without patch:
    ===============================================
     # echo "253:22358 100000000" > /sys/fs/cgroup/blkio/machine.slice/1/blkio.throttle.write_bps_device
     # vzctl exec 1 dd if=/dev/zero of=/test bs=1M count=1000
     # 1048576000 bytes (1.0 GB, 1000 MiB) copied, 1.35522 s, 774 MB/s
    
    Since dirty balancing is global, Container can dirty more than it's RAM
    and blkio limits are not respected.
    
    With patch:
    ===========
     # echo "253:22765 100000000" > /sys/fs/cgroup/blkio/machine.slice/1/blkio.throttle.write_bps_device
     # vzctl exec 1 dd if=/dev/zero of=/test bs=1M count=1000
     # 1048576000 bytes (1.0 GB, 1000 MiB) copied, 10.2267 s, 103 MB/s
    
    Per-ve dirty balancing and throttling work as expected.
    
    v2:
    Since ve->ve_ns is pointing to task nsproxy, it can be changed during ve
    lifetime. We already have a helper ve_get_init_css() that handles this
    case, so I decided to reuse it's code in new cgroup_get_e_ve_css().
    
    Additionally I have added two patches that improve current code:
     1) drop 'get' from css_get_local_root() name since get with css functions
        usually results in taking reference
     2) drop duplicate code and reuse css_local_root() helper in
        ve_get_init_css()
    
    Andrey Zhadchenko (4):
      kernel/cgroup: rename css_get_local_root
      kernel/ve: simplify ve_get_init_css
      kernel/cgroup: implement cgroup_get_e_ve_css
      mm/backing-dev: associate writeback with correct blkcg
---
 kernel/ve/ve.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
index dc920cdf5921..99aac6f6386b 100644
--- a/kernel/ve/ve.c
+++ b/kernel/ve/ve.c
@@ -392,29 +392,22 @@ const char *ve_get_release_agent_path(struct ve_struct *ve,
 
 struct cgroup_subsys_state *ve_get_init_css(struct ve_struct *ve, int subsys_id)
 {
-	struct cgroup_subsys_state *css, *tmp;
-	struct cgroup *cgroup;
+	struct cgroup_subsys_state *css;
 	struct css_set *root_cset;
 	struct nsproxy *nsproxy;
 
 	rcu_read_lock();
 
-	nsproxy = ve->ve_ns ? : &init_nsproxy;
+	nsproxy = rcu_dereference(ve->ve_ns);
+	if (!nsproxy)
+		nsproxy = &init_nsproxy;
+
 	root_cset = nsproxy->cgroup_ns->root_cset;
 	css = root_cset->subsys[subsys_id];
 	/* nsproxy->cgroup_ns must hold root_cset refcnt */
-	BUG_ON(!css_tryget(css));
 
-	cgroup = css->cgroup;
-	while (!test_bit(CGRP_VE_ROOT, &cgroup->flags) && cgroup_parent(cgroup))
-		cgroup = cgroup_parent(cgroup);
-
-	if (cgroup != css->cgroup) {
-		tmp = cgroup->subsys[subsys_id];
-		css_get(tmp);
-		css_put(css);
-		css = tmp;
-	}
+	css = css_local_root(css);
+	BUG_ON(!css_tryget(css));
 
 	rcu_read_unlock();
 	return css;