Message-ID: <CAOUHufbDhqSgSYZwkEo1aF1iFqGge_8jY3dt3OfPwXU0s07KOA@mail.gmail.com>
Date:   Thu, 17 Aug 2023 17:12:17 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] workingset: ensure memcg is valid for recency check

On Thu, Aug 17, 2023 at 4:50 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Thu, Aug 17, 2023 at 3:43 PM Nhat Pham <nphamcs@...il.com> wrote:
> >
> > On Thu, Aug 17, 2023 at 1:50 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > >
> > > On Thu, Aug 17, 2023 at 12:01 PM Nhat Pham <nphamcs@...il.com> 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:
> > > >
> > > > [ 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")
> > >
> > > Beyond mem_cgroup_get_nr_swap_pages(), we still use the eviction_memcg
> > > without grabbing a ref to it first in workingset_test_recent() (and in
> > > workingset_refault() before that) as well as lru_gen_test_recent().
> > >
> > > Wouldn't the fix go back even further? or am I misinterpreting the problem?
> > Hmm I don't see eviction_memcg being used outside of *_test_recent
> > (the rest just uses memcg = folio_memcg(folio), which if I'm not mistaken is
> > the memcg that is refaulting the folio into memory).
> >
> > Inside workingset_test_recent(), the only other place where eviction_memcg
> > is used is for mem_cgroup_lruvec. This function call won't crash whether
> > eviction_memcg is valid or not.
>
> If eviction_memcg is invalid because the memory was already freed, we
> are basically dereferencing garbage in mem_cgroup_lruvec() aren't we?
>
> > The crash only happens during
> > mem_cgroup_get_nr_swap_pages, which has an upward traversal from
> > eviction_memcg to root.
> >
> > Let me know if this does not make sense and/or is insufficient to ensure
> > safe upward traversal from eviction_memcg to root!
> > >
> > >
> > >
> > > > Signed-off-by: Nhat Pham <nphamcs@...il.com>
> > > > Cc: stable@...r.kernel.org
> > > > ---
> > > >  mm/workingset.c | 65 ++++++++++++++++++++++++++++++++-----------------
> > > >  1 file changed, 43 insertions(+), 22 deletions(-)
> > > >
> > > > diff --git a/mm/workingset.c b/mm/workingset.c
> > > > index da58a26d0d4d..03cadad4e484 100644
> > > > --- a/mm/workingset.c
> > > > +++ b/mm/workingset.c
> > > > @@ -206,10 +206,19 @@ 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)
> > > > +/*
> > > > + * Unpacks the stored fields of a shadow entry into the given pointers.
> > > > + *
> > > > + * The memcg pointer is only populated if the memcg recorded in the shadow
> > > > + * entry is valid. In this case, a reference to the memcg will be acquired,
> > > > + * and a corresponding mem_cgroup_put() will be needed when we no longer
> > > > + * need the memcg.
> > > > + */
> > > > +static void unpack_shadow(void *shadow, struct mem_cgroup **memcgp,
> > > > +                       pg_data_t **pgdat, unsigned long *evictionp, bool *workingsetp)
> > > >  {
> > > >         unsigned long entry = xa_to_value(shadow);
> > > > +       struct mem_cgroup *memcg;
> > > >         int memcgid, nid;
> > > >         bool workingset;
> > > >
> > > > @@ -220,7 +229,24 @@ 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 +288,16 @@ 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;
> > >
> > > +Yu Zhao
> > >
> > > There is a change of behavior here, right?
> > >
> > > The existing code will continue if !mem_cgroup_disabled() && !memcg is
> > > true, and mem_cgroup_lruvec() will return the lruvec of the root
> > > memcg. Now we are just returning false.
> > >
> > > Is this intentional?
> > Oh right, there is. Should have cc-ed Yu Zhao as well, my bad.
> > get_maintainers.pl isn't always sufficient I guess :)
> >
> > But yeah, this behavioral change is intentional.
> >
> > Correct me if I'm wrong of course, but it seems like MGLRU should
> > follow the same pattern here. That is, once we return from unpack_shadow,
> > the possible scenarios are the same as prescribed in workingset_test_recent:
> >
> > 1. If mem_cgroup is disabled, we can ignore this check.
> > 2. If mem_cgroup is enabled, then the only reason why we get NULL
> > memcg from unpack_shadow is if the eviction_memcg is no longer
> > valid.  We should not try to get its lruvec, or substitute it with the
> > root memcg, but return false right away (i.e not recent).
> > >
>
> I will leave this for Yu :)

Thanks, Yosry.

Hi Nhat, it seems unnecessary to me to introduce a get/put into
lru_gen_test_recent() because it doesn't suffer from the bug this
patch tries to fix. In theory, the extra get/put can impact
performance, though admittedly the impact is unlikely to be
measurable. Regardless, the general practice is to fix the bug
locally, i.e., when the mem_cgroup_get_nr_swap_pages() path is taken,
rather than change the unrelated path. Thank you.

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