Re: [PATCH] list: kill list_force_poison()

[Eryu Guan]

On Tue, Mar 01, 2016 at 01:44:32PM -0800, Dan Williams wrote:
> Given we have uninitialized list_heads being passed to list_add() it
> will always be the case that those uninitialized values randomly trigger
> the poison value.  Especially since a list_add() operation will seed the
> stack with the poison value for later stack allocations to trip over.
> For example, see these two false positive reports:
> 
>  list_add attempted on force-poisoned entry
>  WARNING: at lib/list_debug.c:34
>  [..]
>  NIP [c00000000043c390] __list_add+0xb0/0x150
>  LR [c00000000043c38c] __list_add+0xac/0x150
>  Call Trace:
>  [c000000fb5fc3320] [c00000000043c38c] __list_add+0xac/0x150 (unreliable)
>  [c000000fb5fc33a0] [c00000000081b454] __down+0x4c/0xf8
>  [c000000fb5fc3410] [c00000000010b6f8] down+0x68/0x70
>  [c000000fb5fc3450] [d0000000201ebf4c] xfs_buf_lock+0x4c/0x150 [xfs]
> 
>  list_add attempted on force-poisoned entry(0000000000000500),
>   new->next == d0000000059ecdb0, new->prev == 0000000000000500
>  WARNING: at lib/list_debug.c:33
>  [..]
>  NIP [c00000000042db78] __list_add+0xa8/0x140
>  LR [c00000000042db74] __list_add+0xa4/0x140
>  Call Trace:
>  [c0000004c749f620] [c00000000042db74] __list_add+0xa4/0x140 (unreliable)
>  [c0000004c749f6b0] [c0000000008010ec] rwsem_down_read_failed+0x6c/0x1a0
>  [c0000004c749f760] [c000000000800828] down_read+0x58/0x60
>  [c0000004c749f7e0] [d000000005a1a6bc] xfs_log_commit_cil+0x7c/0x600 [xfs]
> 
> Reported-by: Eryu Guan <eguan@xxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: <xfs@xxxxxxxxxxx>
> Fixes: commit 5c2c2587b132 ("mm, dax, pmem: introduce {get|put}_dev_pagemap() for dax-gup")
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

With this patch applied, I don't see the warning after 1000 iterations
(of course, the warning has been removed :-)).

Thanks!
Eryu

P.S.
With the RFC patch posted eariler, warnings are still triggered.

> ---
>  include/linux/list.h |   11 -----------
>  kernel/memremap.c    |    9 +++++++--
>  lib/list_debug.c     |    9 ---------
>  3 files changed, 7 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 30cf4200ab40..5356f4d661a7 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -113,17 +113,6 @@ extern void __list_del_entry(struct list_head *entry);
>  extern void list_del(struct list_head *entry);
>  #endif
>  
> -#ifdef CONFIG_DEBUG_LIST
> -/*
> - * See devm_memremap_pages() which wants DEBUG_LIST=y to assert if one
> - * of the pages it allocates is ever passed to list_add()
> - */
> -extern void list_force_poison(struct list_head *entry);
> -#else
> -/* fallback to the less strict LIST_POISON* definitions */
> -#define list_force_poison list_del
> -#endif
> -
>  /**
>   * list_replace - replace old entry by new one
>   * @old : the element to be replaced
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index b981a7b023f0..778191e3e887 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -351,8 +351,13 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
>  	for_each_device_pfn(pfn, page_map) {
>  		struct page *page = pfn_to_page(pfn);
>  
> -		/* ZONE_DEVICE pages must never appear on a slab lru */
> -		list_force_poison(&page->lru);
> +		/*
> +		 * ZONE_DEVICE pages union ->lru with a ->pgmap back
> +		 * pointer.  It is a bug if a ZONE_DEVICE page is ever
> +		 * freed or placed on a driver-private list.  Seed the
> +		 * storage with LIST_POISON* values.
> +		 */
> +		list_del(&page->lru);
>  		page->pgmap = pgmap;
>  	}
>  	devres_add(dev, page_map);
> diff --git a/lib/list_debug.c b/lib/list_debug.c
> index 3345a089ef7b..3859bf63561c 100644
> --- a/lib/list_debug.c
> +++ b/lib/list_debug.c
> @@ -12,13 +12,6 @@
>  #include <linux/kernel.h>
>  #include <linux/rculist.h>
>  
> -static struct list_head force_poison;
> -void list_force_poison(struct list_head *entry)
> -{
> -	entry->next = &force_poison;
> -	entry->prev = &force_poison;
> -}
> -
>  /*
>   * Insert a new entry between two known consecutive entries.
>   *
> @@ -30,8 +23,6 @@ void __list_add(struct list_head *new,
>  			      struct list_head *prev,
>  			      struct list_head *next)
>  {
> -	WARN(new->next == &force_poison || new->prev == &force_poison,
> -		"list_add attempted on force-poisoned entry\n");
>  	WARN(next->prev != prev,
>  		"list_add corruption. next->prev should be "
>  		"prev (%p), but was %p. (next=%p).\n",
>