Re: [PATCH] workingset: ensure memcg is valid for recency check

[Yosry Ahmed]

On Thu, Aug 17, 2023 at 11:13 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> 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.

I see, is this basically protecting against a race with
mem_cgroup_id_put_many()? My thought was that given that we have an
id, then we should have a ref to the memcg, but if we race with
mem_cgroup_id_put_many() we might get the id just before it is removed
and the ref is put, and then use the memcg after it's freed.

Is this what's happening here? If yes, I think this makes sense.

>
> 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 don't want to be nitpicking, so no strong opinions here :)

> >
> > 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.
> >

Fair enough, I guess basic testing with debugging enabled should spot
any misuse quickly enough.