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Date: Fri, 30 Sep 2022 13:25:54 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Feng Tang <feng.tang@...el.com>, Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/slub: fix a slab missed to be freed problem
On 9/30/22 12:07, Feng Tang wrote:
> When enable kasan and kfence's in-kernel kunit test with slub_debug on,
> it caught a problem (in linux-next tree):
>
> ------------[ cut here ]------------
> kmem_cache_destroy test: Slab cache still has objects when called from test_exit+0x1a/0x30
> WARNING: CPU: 3 PID: 240 at mm/slab_common.c:492 kmem_cache_destroy+0x16c/0x170
Assuming the warning was preceded by some kunit test failures?
I don't see how leaving more empty slabs on free list than needed would
cause this warning, the shutdown should just drop the empty slab.
> Modules linked in:
> CPU: 3 PID: 240 Comm: kunit_try_catch Tainted: G B N 6.0.0-rc7-next-20220929 #52
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> RIP: 0010:kmem_cache_destroy+0x16c/0x170
> Code: 41 5c 41 5d e9 a5 04 0b 00 c3 cc cc cc cc 48 8b 55 60 48 8b 4c 24 20 48 c7 c6 40 37 d2 82 48 c7 c7 e8 a0 33 83 e8 4e d7 14 01 <0f> 0b eb a7 41 56 41 89 d6 41 55 49 89 f5 41 54 49 89 fc 55 48 89
> RSP: 0000:ffff88800775fea0 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: ffffffff83bdec48 RCX: 0000000000000000
> RDX: 0000000000000001 RSI: 1ffff11000eebf9e RDI: ffffed1000eebfc6
> RBP: ffff88804362fa00 R08: ffffffff81182e58 R09: ffff88800775fbdf
> R10: ffffed1000eebf7b R11: 0000000000000001 R12: 000000008c800d00
> R13: ffff888005e78040 R14: 0000000000000000 R15: ffff888005cdfad0
> FS: 0000000000000000(0000) GS:ffff88807ed00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 000000000360e001 CR4: 0000000000370ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> <TASK>
> test_exit+0x1a/0x30
> kunit_try_run_case+0xad/0xc0
> kunit_generic_run_threadfn_adapter+0x26/0x50
> kthread+0x17b/0x1b0
>
> It was biscted to commit c7323a5ad078 ("mm/slub: restrict sysfs
> validation to debug caches and make it safe")
>
> The problem is inside free_debug_processing(), in one path, the slab
> on partial list is missed to be freed when partial list is full.
>
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
>
> Hi reviewers,
>
> Sorry for the late reporting, but it's curious that this problem didn't
> show up in my earlier test (which caught some other problems).
I think we can reuse the slab_free and don't need a new bool?
diff --git a/mm/slub.c b/mm/slub.c
index 5c3c31a154ba..a63953f649ed 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2886,22 +2886,25 @@ static noinline void free_debug_processing(
set_freepointer(s, tail, prior);
slab->freelist = head;
- /* Do we need to remove the slab from full or partial list? */
+ /*
+ * If the slab is empty, and node's partial list is full,
+ * it should be discarded anyway no matter it's on full or
+ * partial list.
+ */
+ if (slab->inuse == 0 && n->nr_partial >= s->min_partial)
+ slab_free = slab;
+
if (!prior) {
+ /* was on full list */
remove_full(s, n, slab);
- } else if (slab->inuse == 0 &&
- n->nr_partial >= s->min_partial) {
+ if (!slab_free) {
+ add_partial(n, slab, DEACTIVATE_TO_TAIL);
+ stat(s, FREE_ADD_PARTIAL);
+ }
+ } else if (slab_free) {
remove_partial(n, slab);
stat(s, FREE_REMOVE_PARTIAL);
}
-
- /* Do we need to discard the slab or add to partial list? */
- if (slab->inuse == 0 && n->nr_partial >= s->min_partial) {
- slab_free = slab;
- } else if (!prior) {
- add_partial(n, slab, DEACTIVATE_TO_TAIL);
- stat(s, FREE_ADD_PARTIAL);
- }
}
if (slab_free) {
> mm/slub.c | 28 +++++++++++++++++-----------
> 1 file changed, 17 insertions(+), 11 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 5c3c31a154ba..4c037bd0b22b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2880,28 +2880,34 @@ static noinline void free_debug_processing(
> out:
> if (checks_ok) {
> void *prior = slab->freelist;
> + bool slab_need_discard = false;
>
> /* Perform the actual freeing while we still hold the locks */
> slab->inuse -= cnt;
> set_freepointer(s, tail, prior);
> slab->freelist = head;
>
> - /* Do we need to remove the slab from full or partial list? */
> + /*
> + * If the slab is empty, and node's partial list is full,
> + * it should be discarded anyway no matter it's on full or
> + * partial list.
> + */
> + if (slab->inuse == 0 && n->nr_partial >= s->min_partial) {
> + slab_need_discard = true;
> + slab_free = slab;
> + }
> +
> if (!prior) {
> + /* was on full list */
> remove_full(s, n, slab);
> - } else if (slab->inuse == 0 &&
> - n->nr_partial >= s->min_partial) {
> + if (!slab_need_discard) {
> + add_partial(n, slab, DEACTIVATE_TO_TAIL);
> + stat(s, FREE_ADD_PARTIAL);
> + }
> + } else if (slab_need_discard) {
> remove_partial(n, slab);
> stat(s, FREE_REMOVE_PARTIAL);
> }
> -
> - /* Do we need to discard the slab or add to partial list? */
> - if (slab->inuse == 0 && n->nr_partial >= s->min_partial) {
> - slab_free = slab;
> - } else if (!prior) {
> - add_partial(n, slab, DEACTIVATE_TO_TAIL);
> - stat(s, FREE_ADD_PARTIAL);
> - }
> }
>
> if (slab_free) {
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