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Date: Wed, 27 Oct 2021 10:32:46 -0400
From: Dennis Zhou <dennis@...nel.org>
To: Yuanzheng Song <songyuanzheng@...wei.com>
Cc: akpm@...ux-foundation.org, dennis@...nel.org, tj@...nel.org,
cl@...ux.com, cl@...two.de, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] mm/percpu: fix data-race with
pcpu_nr_empty_pop_pages
Hello,
On Tue, Oct 26, 2021 at 08:43:12AM +0000, Yuanzheng Song wrote:
> When reading the pcpu_nr_empty_pop_pages in pcpu_alloc()
> and writing the pcpu_nr_empty_pop_pages in
> pcpu_update_empty_pages() at the same time,
> the data-race occurs.
>
> ===========
> read-write to 0xffffffff882fdd4c of 4 bytes by task 9424 on cpu 0:
> pcpu_update_empty_pages
> pcpu_chunk_populated
> pcpu_balance_populated
> pcpu_balance_workfn
> process_one_work
> worker_thread
> kthread
> ret_from_fork
>
> read to 0xffffffff882fdd4c of 4 bytes by task 9386 on cpu 3:
> pcpu_alloc
> __alloc_percpu_gfp
> fib_nh_common_init
> fib_nh_init
> fib_create_info
> fib_table_insert
> fib_magic
> ......
> sock_sendmsg_nosec
> sock_sendmsg
> __sys_sendto
> __do_sys_sendto
> __se_sys_sendto
> __x64_sys_sendto
> do_syscall_64
> entry_SYSCALL_64_after_hwframe
> ============
>
> The same problem will occur in these functions:
> pcpu_reclaim_populated(), pcpu_update_empty_pages(),
> pcpu_isolate_chunk().
>
This isn't true. pcpu_nr_empty_pop_pages is write protected by the
pcpu_lock. Both pcpu_reclaim_populated() and pcpu_isolate_chunk()
require holding the pcpu_lock. pcpu_update_empty_pages() doesn't require
holding the lock because it is called by first chunk init code. The
other callers of the function do hold pcpu_lock.
> Using atomic variable operations to slove this
> concurrent access problem.
I apologize, but it takes me a little bit of time to think about these
changes. In this case, I think it's overkill to make it an atomic for a
read only access that doesn't actually matter.
I think a simpler solution is to move the access up after the area_found
label and do this logic behind the pcpu_lock. The code that follows to
populate non-atomic not-backed allocations will never add to the empty
page count. So something like a `bool need_balance` in free_percpu().
The atomic change I think would make more sense if we had a use case of
someone not in the percpu subsystem reading this value, which wouldn't
be good.
Thanks,
Dennis
>
> Signed-off-by: Yuanzheng Song <songyuanzheng@...wei.com>
> ---
> Changes since v1:
> - Change the pcpu_nr_empty_pop_pages to atomic variable
> suggested by Christoph Lameter
>
> mm/percpu-internal.h | 2 +-
> mm/percpu-stats.c | 2 +-
> mm/percpu-vm.c | 2 +-
> mm/percpu.c | 18 ++++++++++--------
> 4 files changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h
> index 639662c20c82..02fbc11ef68a 100644
> --- a/mm/percpu-internal.h
> +++ b/mm/percpu-internal.h
> @@ -72,7 +72,7 @@ extern struct list_head *pcpu_chunk_lists;
> extern int pcpu_nr_slots;
> extern int pcpu_sidelined_slot;
> extern int pcpu_to_depopulate_slot;
> -extern int pcpu_nr_empty_pop_pages;
> +extern atomic_t pcpu_nr_empty_pop_pages;
>
> extern struct pcpu_chunk *pcpu_first_chunk;
> extern struct pcpu_chunk *pcpu_reserved_chunk;
> diff --git a/mm/percpu-stats.c b/mm/percpu-stats.c
> index c6bd092ff7a3..7bd601fd3b6b 100644
> --- a/mm/percpu-stats.c
> +++ b/mm/percpu-stats.c
> @@ -188,7 +188,7 @@ static int percpu_stats_show(struct seq_file *m, void *v)
> PU(nr_max_chunks);
> PU(min_alloc_size);
> PU(max_alloc_size);
> - P("empty_pop_pages", pcpu_nr_empty_pop_pages);
> + P("empty_pop_pages", atomic_read(&pcpu_nr_empty_pop_pages));
> seq_putc(m, '\n');
>
> #undef PU
> diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
> index 2054c9213c43..12b2342448f4 100644
> --- a/mm/percpu-vm.c
> +++ b/mm/percpu-vm.c
> @@ -404,7 +404,7 @@ static bool pcpu_should_reclaim_chunk(struct pcpu_chunk *chunk)
> * chunk, move it to the to_depopulate list.
> */
> return ((chunk->isolated && chunk->nr_empty_pop_pages) ||
> - (pcpu_nr_empty_pop_pages >
> + (atomic_read(&pcpu_nr_empty_pop_pages) >
> (PCPU_EMPTY_POP_PAGES_HIGH + chunk->nr_empty_pop_pages) &&
> chunk->nr_empty_pop_pages >= chunk->nr_pages / 4));
> }
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 293009cc03ef..0c55a25059a2 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -181,7 +181,7 @@ static LIST_HEAD(pcpu_map_extend_chunks);
> * The number of empty populated pages, protected by pcpu_lock.
> * The reserved chunk doesn't contribute to the count.
> */
> -int pcpu_nr_empty_pop_pages;
> +atomic_t pcpu_nr_empty_pop_pages = ATOMIC_INIT(0);
>
> /*
> * The number of populated pages in use by the allocator, protected by
> @@ -574,7 +574,7 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk)
>
> if (!chunk->isolated) {
> chunk->isolated = true;
> - pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages;
> + atomic_sub(chunk->nr_empty_pop_pages, &pcpu_nr_empty_pop_pages);
> }
> list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]);
> }
> @@ -585,7 +585,7 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk)
>
> if (chunk->isolated) {
> chunk->isolated = false;
> - pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages;
> + atomic_add(chunk->nr_empty_pop_pages, &pcpu_nr_empty_pop_pages);
> pcpu_chunk_relocate(chunk, -1);
> }
> }
> @@ -603,7 +603,7 @@ static inline void pcpu_update_empty_pages(struct pcpu_chunk *chunk, int nr)
> {
> chunk->nr_empty_pop_pages += nr;
> if (chunk != pcpu_reserved_chunk && !chunk->isolated)
> - pcpu_nr_empty_pop_pages += nr;
> + atomic_add(nr, &pcpu_nr_empty_pop_pages);
> }
>
> /*
> @@ -1874,7 +1874,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
> mutex_unlock(&pcpu_alloc_mutex);
> }
>
> - if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
> + if (atomic_read(&pcpu_nr_empty_pop_pages) < PCPU_EMPTY_POP_PAGES_LOW)
> pcpu_schedule_balance_work();
>
> /* clear the areas and return address relative to base address */
> @@ -2062,7 +2062,7 @@ static void pcpu_balance_populated(void)
> pcpu_atomic_alloc_failed = false;
> } else {
> nr_to_pop = clamp(PCPU_EMPTY_POP_PAGES_HIGH -
> - pcpu_nr_empty_pop_pages,
> + atomic_read(&pcpu_nr_empty_pop_pages),
> 0, PCPU_EMPTY_POP_PAGES_HIGH);
> }
>
> @@ -2163,7 +2163,8 @@ static void pcpu_reclaim_populated(void)
> break;
>
> /* reintegrate chunk to prevent atomic alloc failures */
> - if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH) {
> + if (atomic_read(&pcpu_nr_empty_pop_pages) <
> + PCPU_EMPTY_POP_PAGES_HIGH) {
> reintegrate = true;
> goto end_chunk;
> }
> @@ -2765,7 +2766,8 @@ void __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
>
> /* link the first chunk in */
> pcpu_first_chunk = chunk;
> - pcpu_nr_empty_pop_pages = pcpu_first_chunk->nr_empty_pop_pages;
> + atomic_set(&pcpu_nr_empty_pop_pages,
> + pcpu_first_chunk->nr_empty_pop_pages);
> pcpu_chunk_relocate(pcpu_first_chunk, -1);
>
> /* include all regions of the first chunk */
> --
> 2.25.1
>
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