Re: [PATCH -next v4] mm/hotplug: silence a lockdep splat with printk()
From: David Hildenbrand
Date: Fri Jan 17 2020 - 03:51:19 EST
On 17.01.20 03:21, Qian Cai wrote:
> It is not that hard to trigger lockdep splats by calling printk from
> under zone->lock. Most of them are false positives caused by lock chains
> introduced early in the boot process and they do not cause any real
> problems (although some of the early boot lock dependencies could
> happenn after boot as well). There are some console drivers which do
s/happenn/happen/
> allocate from the printk context as well and those should be fixed. In
> any case false positives are not that trivial to workaround and it is
> far from optimal to lose lockdep functionality for something that is a
> non-issue.
>
> So change has_unmovable_pages() so that it no longer calls dump_page()
> itself - instead it returns a "struct page *" of the unmovable page back
> to the caller so that in the case of a has_unmovable_pages() failure,
> the caller can call dump_page() after releasing zone->lock. Also, make
> dump_page() is able to report a CMA page as well, so the reason string
> from has_unmovable_pages() can be removed.
>
> Even though has_unmovable_pages doesn't hold any reference to the
> returned page this should be reasonably safe for the purpose of
> reporting the page (dump_page) because it cannot be hotremoved. The
This is only true in the context of memory unplug, but not in the
context of is_mem_section_removable()-> is_pageblock_removable_nolock().
But well, that function is already racy as hell (and I dislike it very
much) :)
> state of the page might change but that is the case even with the
> existing code as zone->lock only plays role for free pages.
>
> While at it, remove a similar but unnecessary debug-only printk() as
> well.
>
> WARNING: possible circular locking dependency detected
> ------------------------------------------------------
> test.sh/8653 is trying to acquire lock:
> ffffffff865a4460 (console_owner){-.-.}, at:
> console_unlock+0x207/0x750
>
> but task is already holding lock:
> ffff88883fff3c58 (&(&zone->lock)->rlock){-.-.}, at:
> __offline_isolated_pages+0x179/0x3e0
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #3 (&(&zone->lock)->rlock){-.-.}:
> __lock_acquire+0x5b3/0xb40
> lock_acquire+0x126/0x280
> _raw_spin_lock+0x2f/0x40
> rmqueue_bulk.constprop.21+0xb6/0x1160
> get_page_from_freelist+0x898/0x22c0
> __alloc_pages_nodemask+0x2f3/0x1cd0
> alloc_pages_current+0x9c/0x110
> allocate_slab+0x4c6/0x19c0
> new_slab+0x46/0x70
> ___slab_alloc+0x58b/0x960
> __slab_alloc+0x43/0x70
> __kmalloc+0x3ad/0x4b0
> __tty_buffer_request_room+0x100/0x250
> tty_insert_flip_string_fixed_flag+0x67/0x110
> pty_write+0xa2/0xf0
> n_tty_write+0x36b/0x7b0
> tty_write+0x284/0x4c0
> __vfs_write+0x50/0xa0
> vfs_write+0x105/0x290
> redirected_tty_write+0x6a/0xc0
> do_iter_write+0x248/0x2a0
> vfs_writev+0x106/0x1e0
> do_writev+0xd4/0x180
> __x64_sys_writev+0x45/0x50
> do_syscall_64+0xcc/0x76c
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> #2 (&(&port->lock)->rlock){-.-.}:
> __lock_acquire+0x5b3/0xb40
> lock_acquire+0x126/0x280
> _raw_spin_lock_irqsave+0x3a/0x50
> tty_port_tty_get+0x20/0x60
> tty_port_default_wakeup+0xf/0x30
> tty_port_tty_wakeup+0x39/0x40
> uart_write_wakeup+0x2a/0x40
> serial8250_tx_chars+0x22e/0x440
> serial8250_handle_irq.part.8+0x14a/0x170
> serial8250_default_handle_irq+0x5c/0x90
> serial8250_interrupt+0xa6/0x130
> __handle_irq_event_percpu+0x78/0x4f0
> handle_irq_event_percpu+0x70/0x100
> handle_irq_event+0x5a/0x8b
> handle_edge_irq+0x117/0x370
> do_IRQ+0x9e/0x1e0
> ret_from_intr+0x0/0x2a
> cpuidle_enter_state+0x156/0x8e0
> cpuidle_enter+0x41/0x70
> call_cpuidle+0x5e/0x90
> do_idle+0x333/0x370
> cpu_startup_entry+0x1d/0x1f
> start_secondary+0x290/0x330
> secondary_startup_64+0xb6/0xc0
>
> -> #1 (&port_lock_key){-.-.}:
> __lock_acquire+0x5b3/0xb40
> lock_acquire+0x126/0x280
> _raw_spin_lock_irqsave+0x3a/0x50
> serial8250_console_write+0x3e4/0x450
> univ8250_console_write+0x4b/0x60
> console_unlock+0x501/0x750
> vprintk_emit+0x10d/0x340
> vprintk_default+0x1f/0x30
> vprintk_func+0x44/0xd4
> printk+0x9f/0xc5
>
> -> #0 (console_owner){-.-.}:
> check_prev_add+0x107/0xea0
> validate_chain+0x8fc/0x1200
> __lock_acquire+0x5b3/0xb40
> lock_acquire+0x126/0x280
> console_unlock+0x269/0x750
> vprintk_emit+0x10d/0x340
> vprintk_default+0x1f/0x30
> vprintk_func+0x44/0xd4
> printk+0x9f/0xc5
> __offline_isolated_pages.cold.52+0x2f/0x30a
> offline_isolated_pages_cb+0x17/0x30
> walk_system_ram_range+0xda/0x160
> __offline_pages+0x79c/0xa10
> offline_pages+0x11/0x20
> memory_subsys_offline+0x7e/0xc0
> device_offline+0xd5/0x110
> state_store+0xc6/0xe0
> dev_attr_store+0x3f/0x60
> sysfs_kf_write+0x89/0xb0
> kernfs_fop_write+0x188/0x240
> __vfs_write+0x50/0xa0
> vfs_write+0x105/0x290
> ksys_write+0xc6/0x160
> __x64_sys_write+0x43/0x50
> do_syscall_64+0xcc/0x76c
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> other info that might help us debug this:
>
> Chain exists of:
> console_owner --> &(&port->lock)->rlock --> &(&zone->lock)-
>
>> rlock
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&(&zone->lock)->rlock);
> lock(&(&port->lock)->rlock);
> lock(&(&zone->lock)->rlock);
> lock(console_owner);
>
> *** DEADLOCK ***
>
> 9 locks held by test.sh/8653:
> #0: ffff88839ba7d408 (sb_writers#4){.+.+}, at:
> vfs_write+0x25f/0x290
> #1: ffff888277618880 (&of->mutex){+.+.}, at:
> kernfs_fop_write+0x128/0x240
> #2: ffff8898131fc218 (kn->count#115){.+.+}, at:
> kernfs_fop_write+0x138/0x240
> #3: ffffffff86962a80 (device_hotplug_lock){+.+.}, at:
> lock_device_hotplug_sysfs+0x16/0x50
> #4: ffff8884374f4990 (&dev->mutex){....}, at:
> device_offline+0x70/0x110
> #5: ffffffff86515250 (cpu_hotplug_lock.rw_sem){++++}, at:
> __offline_pages+0xbf/0xa10
> #6: ffffffff867405f0 (mem_hotplug_lock.rw_sem){++++}, at:
> percpu_down_write+0x87/0x2f0
> #7: ffff88883fff3c58 (&(&zone->lock)->rlock){-.-.}, at:
> __offline_isolated_pages+0x179/0x3e0
> #8: ffffffff865a4920 (console_lock){+.+.}, at:
> vprintk_emit+0x100/0x340
>
> stack backtrace:
> Hardware name: HPE ProLiant DL560 Gen10/ProLiant DL560 Gen10,
> BIOS U34 05/21/2019
> Call Trace:
> dump_stack+0x86/0xca
> print_circular_bug.cold.31+0x243/0x26e
> check_noncircular+0x29e/0x2e0
> check_prev_add+0x107/0xea0
> validate_chain+0x8fc/0x1200
> __lock_acquire+0x5b3/0xb40
> lock_acquire+0x126/0x280
> console_unlock+0x269/0x750
> vprintk_emit+0x10d/0x340
> vprintk_default+0x1f/0x30
> vprintk_func+0x44/0xd4
> printk+0x9f/0xc5
> __offline_isolated_pages.cold.52+0x2f/0x30a
> offline_isolated_pages_cb+0x17/0x30
> walk_system_ram_range+0xda/0x160
> __offline_pages+0x79c/0xa10
> offline_pages+0x11/0x20
> memory_subsys_offline+0x7e/0xc0
> device_offline+0xd5/0x110
> state_store+0xc6/0xe0
> dev_attr_store+0x3f/0x60
> sysfs_kf_write+0x89/0xb0
> kernfs_fop_write+0x188/0x240
> __vfs_write+0x50/0xa0
> vfs_write+0x105/0x290
> ksys_write+0xc6/0x160
> __x64_sys_write+0x43/0x50
> do_syscall_64+0xcc/0x76c
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
>
> v4: Update the commit log again thanks to Michal.
> v3: Rebase to next-20200115 for the mm/debug change and update some
> comments thanks to Michal.
> v2: Improve the commit log and report CMA in dump_page() per Andrew.
> has_unmovable_pages() returns a "struct page *" to the caller.
>
> include/linux/page-isolation.h | 4 ++--
> mm/debug.c | 4 +++-
> mm/memory_hotplug.c | 6 ++++--
> mm/page_alloc.c | 22 +++++++++-------------
> mm/page_isolation.c | 11 ++++++++++-
> 5 files changed, 28 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
> index 148e65a9c606..da043ae86488 100644
> --- a/include/linux/page-isolation.h
> +++ b/include/linux/page-isolation.h
> @@ -33,8 +33,8 @@ static inline bool is_migrate_isolate(int migratetype)
> #define MEMORY_OFFLINE 0x1
> #define REPORT_FAILURE 0x2
>
> -bool has_unmovable_pages(struct zone *zone, struct page *page, int migratetype,
> - int flags);
> +struct page *has_unmovable_pages(struct zone *zone, struct page *page, int
> + migratetype, int flags);
> void set_pageblock_migratetype(struct page *page, int migratetype);
> int move_freepages_block(struct zone *zone, struct page *page,
> int migratetype, int *num_movable);
> diff --git a/mm/debug.c b/mm/debug.c
> index 6a52316af839..784f9da711b0 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -46,6 +46,7 @@ void __dump_page(struct page *page, const char *reason)
> {
> struct address_space *mapping;
> bool page_poisoned = PagePoisoned(page);
> + bool page_cma = is_migrate_cma_page(page);
-> you are accessing the pageblock without the zone lock. It could
change to "isolate" again in the meantime if I am not wrong!
> int mapcount;
> char *type = "";
>
> @@ -92,7 +93,8 @@ void __dump_page(struct page *page, const char *reason)
> }
> BUILD_BUG_ON(ARRAY_SIZE(pageflag_names) != __NR_PAGEFLAGS + 1);
>
> - pr_warn("%sflags: %#lx(%pGp)\n", type, page->flags, &page->flags);
> + pr_warn("%sflags: %#lx(%pGp)%s", type, page->flags, &page->flags,
> + page_cma ? " CMA\n" : "\n");
I'd do a
pr_warn("%sflags: %#lx(%pGp)%s\n", type, page->flags, &page->flags,
page_cma ? " CMA" : "");
>
> hex_only:
> print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 7a6de9b0dcab..06e7dd3eb9a9 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1148,8 +1148,10 @@ static bool is_pageblock_removable_nolock(unsigned long pfn)
> if (!zone_spans_pfn(zone, pfn))
> return false;
>
> - return !has_unmovable_pages(zone, page, MIGRATE_MOVABLE,
> - MEMORY_OFFLINE);
> + if (has_unmovable_pages(zone, page, MIGRATE_MOVABLE, MEMORY_OFFLINE))
> + return false;
> +
> + return true;
if it returns NULL, !NULL converts it to "true"
if it returns PTR, !PTR converts it to "false"
Is this change really necessary?
> }
>
> /* Checks if this range of memory is likely to be hot-removable. */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e56cd1f33242..e90140e879e6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8202,13 +8202,16 @@ void *__init alloc_large_system_hash(const char *tablename,
> * MIGRATE_MOVABLE block might include unmovable pages. And __PageMovable
> * check without lock_page also may miss some movable non-lru pages at
> * race condition. So you can't expect this function should be exact.
> + *
> + * It returns a page without holding a reference. It should be safe here
> + * because the page cannot go away because it is unmovable, but it must not to
> + * be used for anything else rather than dumping its state.
I think something like this would be better:
"Returns a page without holding a reference. If the caller wants to
dereference that page (e.g., dumping), it has to make sure that that it
cannot get removed (e.g., via memory unplug) concurrently."
> */
> -bool has_unmovable_pages(struct zone *zone, struct page *page, int migratetype,
> - int flags)
> +struct page *has_unmovable_pages(struct zone *zone, struct page *page,
> + int migratetype, int flags)
> {
> unsigned long iter = 0;
> unsigned long pfn = page_to_pfn(page);
> - const char *reason = "unmovable page";
>
> /*
> * TODO we could make this much more efficient by not checking every
> @@ -8225,9 +8228,8 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int migratetype,
> * so consider them movable here.
> */
> if (is_migrate_cma(migratetype))
> - return false;
> + return NULL;
>
> - reason = "CMA page";
> goto unmovable;
> }
>
> @@ -8302,12 +8304,10 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int migratetype,
> */
> goto unmovable;
> }
> - return false;
> + return NULL;
> unmovable:
> WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
> - if (flags & REPORT_FAILURE)
> - dump_page(pfn_to_page(pfn + iter), reason);
> - return true;
> + return pfn_to_page(pfn + iter);
> }
>
> #ifdef CONFIG_CONTIG_ALLOC
> @@ -8711,10 +8711,6 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
> BUG_ON(!PageBuddy(page));
> order = page_order(page);
> offlined_pages += 1 << order;
> -#ifdef CONFIG_DEBUG_VM
> - pr_info("remove from free list %lx %d %lx\n",
> - pfn, 1 << order, end_pfn);
> -#endif
> del_page_from_free_area(page, &zone->free_area[order]);
> pfn += (1 << order);
> }
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 1f8b9dfecbe8..f3af65bac1e0 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -20,6 +20,7 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> struct zone *zone;
> unsigned long flags;
> int ret = -EBUSY;
> + struct page *unmovable = NULL;
nit: reverse christmas tree please :) (move it to the top)
>
> zone = page_zone(page);
>
> @@ -37,7 +38,8 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
> * We just check MOVABLE pages.
> */
> - if (!has_unmovable_pages(zone, page, migratetype, isol_flags)) {
> + unmovable = has_unmovable_pages(zone, page, migratetype, isol_flags);
> + if (!unmovable) {
> unsigned long nr_pages;
> int mt = get_pageblock_migratetype(page);
>
> @@ -54,6 +56,13 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> spin_unlock_irqrestore(&zone->lock, flags);
> if (!ret)
> drain_all_pages(zone);
> + else if (isol_flags & REPORT_FAILURE && unmovable)
(isol_flags & REPORT_FAILURE) please for readability
> + /*
> + * printk() with zone->lock held will guarantee to trigger a
> + * lockdep splat, so defer it here.
> + */
> + dump_page(unmovable, "unmovable page");
> +
> return ret;
> }
--
Thanks,
David / dhildenb