Re: [-mm PATCH v2 19/25] list: introduce list_del_poison()

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Dec 15, 2015 at 3:41 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Wed, 09 Dec 2015 18:38:55 -0800 Dan Williams <dan.j.williams@intel.com> wrote:
>
>> ZONE_DEVICE pages always have an elevated count and will never be on an
>> lru reclaim list.  That space in 'struct page' can be redirected for
>> other uses, but for safety introduce a poison value that will always
>> trip __list_add() to assert.  This allows half of the struct list_head
>> storage to be reclaimed with some assurance to back up the assumption
>> that the page count never goes to zero and a list_add() is never
>> attempted.
>>
>> ...
>>
>> --- a/include/linux/list.h
>> +++ b/include/linux/list.h
>> @@ -108,9 +108,26 @@ static inline void list_del(struct list_head *entry)
>>       entry->next = LIST_POISON1;
>>       entry->prev = LIST_POISON2;
>>  }
>> +
>> +#define list_del_poison list_del
>>  #else
>>  extern void __list_del_entry(struct list_head *entry);
>>  extern void list_del(struct list_head *entry);
>> +extern struct list_head list_force_poison;
>> +
>> +/**
>> + * list_del_poison - poison an entry to always assert on list_add
>> + * @entry: the element to delete and poison
>> + *
>> + * Note: the assertion on list_add() only occurs when CONFIG_DEBUG_LIST=y,
>> + * otherwise this is identical to list_del()
>> + */
>> +static inline void list_del_poison(struct list_head *entry)
>> +{
>> +     __list_del(entry->prev, entry->next);
>> +     entry->next = &list_force_poison;
>> +     entry->prev = &list_force_poison;
>> +}
>>  #endif
>
> list_del() already poisons the list_head.  Does this really add anything?

The poison values that list_del() injects detect double-deletion, but
it is fine to re-add a deleted entry.

This version causes list_add() to warn if the list entry is ever
attempted to be added.

Maybe it is better called list_del_permanent(), but this really is a
special case to catch code that attempts to use page->lru for a
zone_device page.  I think I'll make list_del_poison() private to
kernel/memremap.c for now, I don't suspect any other code is doing
union tricks like struct page to re-purpose fields.

>>  /**
>> diff --git a/lib/list_debug.c b/lib/list_debug.c
>> index 3859bf63561c..d730c064a4df 100644
>> --- a/lib/list_debug.c
>> +++ b/lib/list_debug.c
>> @@ -12,6 +12,8 @@
>>  #include <linux/kernel.h>
>>  #include <linux/rculist.h>
>>
>> +struct list_head list_force_poison;
>> +
>>  /*
>>   * Insert a new entry between two known consecutive entries.
>>   *
>> @@ -23,6 +25,8 @@ void __list_add(struct list_head *new,
>>                             struct list_head *prev,
>>                             struct list_head *next)
>>  {
>> +     WARN(new->next == &list_force_poison || new->prev == &list_force_poison,
>> +             "list_add attempted on force-poisoned entry\n");
>
> I suppose that list_replace() should poison as well, and perhaps other
> places were missed.
>

At least in the case of list_replace() it has to have gone through a
list_add() first.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>