Thanks for the review, Yosry!
On Thu, Aug 17, 2023 at 10:40 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Thu, Aug 17, 2023 at 9:47 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >
> > In eviction recency check, we are currently not holding a local
> > reference to the memcg that the refaulted folio belonged to when it was
> > evicted. This could cause serious memcg lifetime issues, for e.g in the
> > memcg hierarchy traversal done in mem_cgroup_get_nr_swap_pages(). This
> > has occurred in production:
>
> Doesn't workingset_refault() call workingset_test_recent() under RCU?
Yep we're under RCU protection. But the eviction_memcg is grabbed
from a memcg id packed in the shadow - its reference count might
go down to 0 in the meantime and get killed.
This pattern (tryget memcg inside RCU) is done in a couple
other places too.
> Shouldn't this guarantee the validity of the memcg?
>
> >
> > [ 155757.793456] BUG: kernel NULL pointer dereference, address: 00000000000000c0
> > [ 155757.807568] #PF: supervisor read access in kernel mode
> > [ 155757.818024] #PF: error_code(0x0000) - not-present page
> > [ 155757.828482] PGD 401f77067 P4D 401f77067 PUD 401f76067 PMD 0
> > [ 155757.839985] Oops: 0000 [#1] SMP
> > [ 155757.846444] CPU: 7 PID: 1380944 Comm: ThriftSrv-pri3- Kdump: loaded Tainted: G S 6.4.3-0_fbk1_rc0_594_g8d0cbcaa67ba #1
> > [ 155757.870808] Hardware name: Wiwynn Twin Lakes MP/Twin Lakes Passive MP, BIOS YMM16 05/24/2021
> > [ 155757.887870] RIP: 0010:mem_cgroup_get_nr_swap_pages+0x3d/0xb0
> > [ 155757.899377] Code: 29 19 4a 02 48 39 f9 74 63 48 8b 97 c0 00 00 00 48 8b b7 58 02 00 00 48 2b b7 c0 01 00 00 48 39 f0 48 0f 4d c6 48 39 d1 74 42 <48> 8b b2 c0 00 00 00 48 8b ba 58 02 00 00 48 2b ba c0 01 00 00 48
> > [ 155757.937125] RSP: 0018:ffffc9002ecdfbc8 EFLAGS: 00010286
> > [ 155757.947755] RAX: 00000000003a3b1c RBX: 000007ffffffffff RCX: ffff888280183000
> > [ 155757.962202] RDX: 0000000000000000 RSI: 0007ffffffffffff RDI: ffff888bbc2d1000
> > [ 155757.976648] RBP: 0000000000000001 R08: 000000000000000b R09: ffff888ad9cedba0
> > [ 155757.991094] R10: ffffea0039c07900 R11: 0000000000000010 R12: ffff888b23a7b000
> > [ 155758.005540] R13: 0000000000000000 R14: ffff888bbc2d1000 R15: 000007ffffc71354
> > [ 155758.019991] FS: 00007f6234c68640(0000) GS:ffff88903f9c0000(0000) knlGS:0000000000000000
> > [ 155758.036356] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 155758.048023] CR2: 00000000000000c0 CR3: 0000000a83eb8004 CR4: 00000000007706e0
> > [ 155758.062473] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > [ 155758.076924] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > [ 155758.091376] PKRU: 55555554
> > [ 155758.096957] Call Trace:
> > [ 155758.102016] <TASK>
> > [ 155758.106502] ? __die+0x78/0xc0
> > [ 155758.112793] ? page_fault_oops+0x286/0x380
> > [ 155758.121175] ? exc_page_fault+0x5d/0x110
> > [ 155758.129209] ? asm_exc_page_fault+0x22/0x30
> > [ 155758.137763] ? mem_cgroup_get_nr_swap_pages+0x3d/0xb0
> > [ 155758.148060] workingset_test_recent+0xda/0x1b0
> > [ 155758.157133] workingset_refault+0xca/0x1e0
> > [ 155758.165508] filemap_add_folio+0x4d/0x70
> > [ 155758.173538] page_cache_ra_unbounded+0xed/0x190
> > [ 155758.182919] page_cache_sync_ra+0xd6/0x1e0
> > [ 155758.191738] filemap_read+0x68d/0xdf0
> > [ 155758.199495] ? mlx5e_napi_poll+0x123/0x940
> > [ 155758.207981] ? __napi_schedule+0x55/0x90
> > [ 155758.216095] __x64_sys_pread64+0x1d6/0x2c0
> > [ 155758.224601] do_syscall_64+0x3d/0x80
> > [ 155758.232058] entry_SYSCALL_64_after_hwframe+0x46/0xb0
> > [ 155758.242473] RIP: 0033:0x7f62c29153b5
> > [ 155758.249938] Code: e8 48 89 75 f0 89 7d f8 48 89 4d e0 e8 b4 e6 f7 ff 41 89 c0 4c 8b 55 e0 48 8b 55 e8 48 8b 75 f0 8b 7d f8 b8 11 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 33 44 89 c7 48 89 45 f8 e8 e7 e6 f7 ff 48 8b
> > [ 155758.288005] RSP: 002b:00007f6234c5ffd0 EFLAGS: 00000293 ORIG_RAX: 0000000000000011
> > [ 155758.303474] RAX: ffffffffffffffda RBX: 00007f628c4e70c0 RCX: 00007f62c29153b5
> > [ 155758.318075] RDX: 000000000003c041 RSI: 00007f61d2986000 RDI: 0000000000000076
> > [ 155758.332678] RBP: 00007f6234c5fff0 R08: 0000000000000000 R09: 0000000064d5230c
> > [ 155758.347452] R10: 000000000027d450 R11: 0000000000000293 R12: 000000000003c041
> > [ 155758.362044] R13: 00007f61d2986000 R14: 00007f629e11b060 R15: 000000000027d450
> > [ 155758.376661] </TASK>
> >
> > This patch fixes the issue by getting a local reference inside
> > unpack_shadow().
> >
> > Fixes: f78dfc7b77d5 ("workingset: fix confusion around eviction vs refault container")
> > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> > mm/workingset.c | 57 ++++++++++++++++++++++++++++++-------------------
> > 1 file changed, 35 insertions(+), 22 deletions(-)
> >
> > diff --git a/mm/workingset.c b/mm/workingset.c
> > index da58a26d0d4d..c20b26bb6cb1 100644
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -206,10 +206,11 @@ static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction,
> > return xa_mk_value(eviction);
> > }
> >
>
> > -static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
> > - unsigned long *evictionp, bool *workingsetp)
> > +static void unpack_shadow(void *shadow, struct mem_cgroup **memcgp,
> > + pg_data_t **pgdat, unsigned long *evictionp, bool *workingsetp)
>
> We should probably document that we are returning a memcg with a ref
> on it, or change the name to put "get" and "memcg" in there somewhere.
> Alternatively we could separate the function that gets the memcg from
> the shadow (e.g. get_memcg_from_shadow or so).
Hmm I feel like unpack as a verb is good here. But I can add a small
comment/documentation above the function to signify that this
function holds a reference to the memcg, and a corresponding
put call will be needed, if that helps.
>
> I would also hold an RCU read lock here so that the call to
> mem_cgroup_from_id() is valid. I know all callers currently do, but it
> may change later. Alternatively we can add an assertion or a warning.
I prefer the assertion idea. But FWIW, mem_cgroup_from_id()
already has a warning for this
(WARN_ON_ONCE(!rcu_read_lock_held()))
Seems redundant to me, especially if the reason why we care
about unpack_shadow() being under RCU is for
mem_cgroup_from_id() to be valid.
>
> > {
> > unsigned long entry = xa_to_value(shadow);
> > + struct mem_cgroup *memcg;
> > int memcgid, nid;
> > bool workingset;
> >
> > @@ -220,7 +221,24 @@ static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
> > memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
> > entry >>= MEM_CGROUP_ID_SHIFT;
> >
> > - *memcgidp = memcgid;
> > + /*
> > + * Look up the memcg associated with the stored ID. It might
> > + * have been deleted since the folio's eviction.
> > + *
> > + * Note that in rare events the ID could have been recycled
> > + * for a new cgroup that refaults a shared folio. This is
> > + * impossible to tell from the available data. However, this
> > + * should be a rare and limited disturbance, and activations
> > + * are always speculative anyway. Ultimately, it's the aging
> > + * algorithm's job to shake out the minimum access frequency
> > + * for the active cache.
> > + */
> > + memcg = mem_cgroup_from_id(memcgid);
> > + if (memcg && css_tryget(&memcg->css))
> > + *memcgp = memcg;
> > + else
> > + *memcgp = NULL;
> > +
> > *pgdat = NODE_DATA(nid);
> > *evictionp = entry;
> > *workingsetp = workingset;
> > @@ -262,15 +280,16 @@ static void *lru_gen_eviction(struct folio *folio)
> > static bool lru_gen_test_recent(void *shadow, bool file, struct lruvec **lruvec,
> > unsigned long *token, bool *workingset)
> > {
> > - int memcg_id;
> > unsigned long min_seq;
> > struct mem_cgroup *memcg;
> > struct pglist_data *pgdat;
> >
> > - unpack_shadow(shadow, &memcg_id, &pgdat, token, workingset);
> > + unpack_shadow(shadow, &memcg, &pgdat, token, workingset);
> > + if (!mem_cgroup_disabled() && !memcg)
> > + return false;
> >
> > - memcg = mem_cgroup_from_id(memcg_id);
> > *lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > + mem_cgroup_put(memcg);
> >
> > min_seq = READ_ONCE((*lruvec)->lrugen.min_seq[file]);
> > return (*token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
> > @@ -421,36 +440,29 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
> > unsigned long refault_distance;
> > unsigned long workingset_size;
> > unsigned long refault;
> > - int memcgid;
> > struct pglist_data *pgdat;
> > unsigned long eviction;
> >
> > if (lru_gen_enabled())
> > return lru_gen_test_recent(shadow, file, &eviction_lruvec, &eviction, workingset);
> >
> > - unpack_shadow(shadow, &memcgid, &pgdat, &eviction, workingset);
> > - eviction <<= bucket_order;
> > -
> > + unpack_shadow(shadow, &eviction_memcg, &pgdat, &eviction, workingset);
> > /*
> > - * Look up the memcg associated with the stored ID. It might
> > - * have been deleted since the folio's eviction.
> > + * When memcg is enabled, we only get !memcg here if the
> > + * eviction group has been deleted. In that case, ignore
> > + * the refault.
> > *
> > - * Note that in rare events the ID could have been recycled
> > - * for a new cgroup that refaults a shared folio. This is
> > - * impossible to tell from the available data. However, this
> > - * should be a rare and limited disturbance, and activations
> > - * are always speculative anyway. Ultimately, it's the aging
> > - * algorithm's job to shake out the minimum access frequency
> > - * for the active cache.
> > + * When memcg is disabled, we always get NULL since there
> > + * is no root_mem_cgroup for !CONFIG_MEMCG. Continue; the
> > + * mem_cgroup_lruvec() will get us the global lruvec.
> > *
> > - * XXX: On !CONFIG_MEMCG, this will always return NULL; it
> > - * would be better if the root_mem_cgroup existed in all
> > + * XXX: It would be better if the root_mem_cgroup existed in all
> > * configurations instead.
> > */
> > - eviction_memcg = mem_cgroup_from_id(memcgid);
> > if (!mem_cgroup_disabled() && !eviction_memcg)
> > return false;
> >
> > + eviction <<= bucket_order;
> > eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
> > refault = atomic_long_read(&eviction_lruvec->nonresident_age);
> >
> > @@ -493,6 +505,7 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
> > }
> > }
> >
> > + mem_cgroup_put(eviction_memcg);
> > return refault_distance <= workingset_size;
> > }
> >
> > --
> > 2.34.1
> >
> >
