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author | Russell King <rmk+kernel@arm.linux.org.uk> | 2013-10-30 14:16:16 +0000 |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2013-10-30 12:57:46 -0700 |
commit | c56b097af26cb11c1f49a4311ba538c825666fed (patch) | |
tree | ca39288548c3e660521fd298cdd8bba9418eb222 | |
parent | ced5d6b552e9ac8dad0e287a5d10c2b620a69e6e (diff) | |
download | op-kernel-dev-c56b097af26cb11c1f49a4311ba538c825666fed.zip op-kernel-dev-c56b097af26cb11c1f49a4311ba538c825666fed.tar.gz |
mm: list_lru: fix almost infinite loop causing effective livelock
I've seen a fair number of issues with kswapd and other processes
appearing to get stuck in v3.12-rc. Using sysrq-p many times seems to
indicate that it gets stuck somewhere in list_lru_walk_node(), called
from prune_icache_sb() and super_cache_scan().
I never seem to be able to trigger a calltrace for functions above that
point.
So I decided to add the following to super_cache_scan():
@@ -81,10 +81,14 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
inodes = list_lru_count_node(&sb->s_inode_lru, sc->nid);
dentries = list_lru_count_node(&sb->s_dentry_lru, sc->nid);
total_objects = dentries + inodes + fs_objects + 1;
+printk("%s:%u: %s: dentries %lu inodes %lu total %lu\n", current->comm, current->pid, __func__, dentries, inodes, total_objects);
/* proportion the scan between the caches */
dentries = mult_frac(sc->nr_to_scan, dentries, total_objects);
inodes = mult_frac(sc->nr_to_scan, inodes, total_objects);
+printk("%s:%u: %s: dentries %lu inodes %lu\n", current->comm, current->pid, __func__, dentries, inodes);
+BUG_ON(dentries == 0);
+BUG_ON(inodes == 0);
/*
* prune the dcache first as the icache is pinned by it, then
@@ -99,7 +103,7 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
freed += sb->s_op->free_cached_objects(sb, fs_objects,
sc->nid);
}
-
+printk("%s:%u: %s: dentries %lu inodes %lu freed %lu\n", current->comm, current->pid, __func__, dentries, inodes, freed);
drop_super(sb);
return freed;
}
and shortly thereafter, having applied some pressure, I got this:
update-apt-xapi:1616: super_cache_scan: dentries 25632 inodes 2 total 25635
update-apt-xapi:1616: super_cache_scan: dentries 1023 inodes 0
------------[ cut here ]------------
Kernel BUG at c0101994 [verbose debug info unavailable]
Internal error: Oops - BUG: 0 [#3] SMP ARM
Modules linked in: fuse rfcomm bnep bluetooth hid_cypress
CPU: 0 PID: 1616 Comm: update-apt-xapi Tainted: G D 3.12.0-rc7+ #154
task: daea1200 ti: c3bf8000 task.ti: c3bf8000
PC is at super_cache_scan+0x1c0/0x278
LR is at trace_hardirqs_on+0x14/0x18
Process update-apt-xapi (pid: 1616, stack limit = 0xc3bf8240)
...
Backtrace:
(super_cache_scan) from [<c00cd69c>] (shrink_slab+0x254/0x4c8)
(shrink_slab) from [<c00d09a0>] (try_to_free_pages+0x3a0/0x5e0)
(try_to_free_pages) from [<c00c59cc>] (__alloc_pages_nodemask+0x5)
(__alloc_pages_nodemask) from [<c00e07c0>] (__pte_alloc+0x2c/0x13)
(__pte_alloc) from [<c00e3a70>] (handle_mm_fault+0x84c/0x914)
(handle_mm_fault) from [<c001a4cc>] (do_page_fault+0x1f0/0x3bc)
(do_page_fault) from [<c001a7b0>] (do_translation_fault+0xac/0xb8)
(do_translation_fault) from [<c000840c>] (do_DataAbort+0x38/0xa0)
(do_DataAbort) from [<c00133f8>] (__dabt_usr+0x38/0x40)
Notice that we had a very low number of inodes, which were reduced to
zero my mult_frac().
Now, prune_icache_sb() calls list_lru_walk_node() passing that number of
inodes (0) into that as the number of objects to scan:
long prune_icache_sb(struct super_block *sb, unsigned long nr_to_scan,
int nid)
{
LIST_HEAD(freeable);
long freed;
freed = list_lru_walk_node(&sb->s_inode_lru, nid, inode_lru_isolate,
&freeable, &nr_to_scan);
which does:
unsigned long
list_lru_walk_node(struct list_lru *lru, int nid, list_lru_walk_cb isolate,
void *cb_arg, unsigned long *nr_to_walk)
{
struct list_lru_node *nlru = &lru->node[nid];
struct list_head *item, *n;
unsigned long isolated = 0;
spin_lock(&nlru->lock);
restart:
list_for_each_safe(item, n, &nlru->list) {
enum lru_status ret;
/*
* decrement nr_to_walk first so that we don't livelock if we
* get stuck on large numbesr of LRU_RETRY items
*/
if (--(*nr_to_walk) == 0)
break;
So, if *nr_to_walk was zero when this function was entered, that means
we're wanting to operate on (~0UL)+1 objects - which might as well be
infinite.
Clearly this is not correct behaviour. If we think about the behaviour
of this function when *nr_to_walk is 1, then clearly it's wrong - we
decrement first and then test for zero - which results in us doing
nothing at all. A post-decrement would give the desired behaviour -
we'd try to walk one object and one object only if *nr_to_walk were one.
It also gives the correct behaviour for zero - we exit at this point.
Fixes: 5cedf721a7cd ("list_lru: fix broken LRU_RETRY behaviour")
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
[ Modified to make sure we never underflow the count: this function gets
called in a loop, so the 0 -> ~0ul transition is dangerous - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r-- | mm/list_lru.c | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/mm/list_lru.c b/mm/list_lru.c index 7246791..72f9dec 100644 --- a/mm/list_lru.c +++ b/mm/list_lru.c @@ -81,8 +81,9 @@ restart: * decrement nr_to_walk first so that we don't livelock if we * get stuck on large numbesr of LRU_RETRY items */ - if (--(*nr_to_walk) == 0) + if (!*nr_to_walk) break; + --*nr_to_walk; ret = isolate(item, &nlru->lock, cb_arg); switch (ret) { |