Re: [RFC v2 7/9] mm/damon: Implement callbacks for physical memory monitoring

[David Hildenbrand]

On 04.06.20 17:23, SeongJae Park wrote:
> On Thu, 4 Jun 2020 16:58:13 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
>> On 04.06.20 09:26, SeongJae Park wrote:
>>> On Wed, 3 Jun 2020 18:09:21 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>
>>>> On 03.06.20 16:11, SeongJae Park wrote:
>>>>> From: SeongJae Park <sjpark@xxxxxxxxx>
>>>>>
>>>>> This commit implements the four callbacks (->init_target_regions,
>>>>> ->update_target_regions, ->prepare_access_check, and ->check_accesses)
>>>>> for the basic access monitoring of the physical memory address space.
>>>>> By setting the callback pointers to point those, users can easily
>>>>> monitor the accesses to the physical memory.
>>>>>
>>>>> Internally, it uses the PTE Accessed bit, as similar to that of the
>>>>> virtual memory support.  Also, it supports only page frames that
>>>>> supported by idle page tracking.  Acutally, most of the code is stollen
>>>>> from idle page tracking.  Users who want to use other access check
>>>>> primitives and monitor the frames that not supported with this
>>>>> implementation could implement their own callbacks on their own.
>>>>>
>>>>> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
>>>>> ---
>>>>>  include/linux/damon.h |   5 ++
>>>>>  mm/damon.c            | 184 ++++++++++++++++++++++++++++++++++++++++++
>>>>>  2 files changed, 189 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/damon.h b/include/linux/damon.h
>>>>> index 1a788bfd1b4e..f96503a532ea 100644
>>>>> --- a/include/linux/damon.h
>>>>> +++ b/include/linux/damon.h
>>>>> @@ -216,6 +216,11 @@ void kdamond_update_vm_regions(struct damon_ctx *ctx);
>>>>>  void kdamond_prepare_vm_access_checks(struct damon_ctx *ctx);
>>>>>  unsigned int kdamond_check_vm_accesses(struct damon_ctx *ctx);
>>>>>  
>>>>> +void kdamond_init_phys_regions(struct damon_ctx *ctx);
>>>>> +void kdamond_update_phys_regions(struct damon_ctx *ctx);
>>>>> +void kdamond_prepare_phys_access_checks(struct damon_ctx *ctx);
>>>>> +unsigned int kdamond_check_phys_accesses(struct damon_ctx *ctx);
>>>>> +
>>>>>  int damon_set_pids(struct damon_ctx *ctx, int *pids, ssize_t nr_pids);
>>>>>  int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
>>>>>  		unsigned long aggr_int, unsigned long regions_update_int,
>>>>> diff --git a/mm/damon.c b/mm/damon.c
>>>>> index f5cbc97a3bbc..6a5c6d540580 100644
>>>>> --- a/mm/damon.c
>>>>> +++ b/mm/damon.c
>>>>> @@ -19,7 +19,9 @@
>>>>>  #include <linux/mm.h>
>>>>>  #include <linux/module.h>
>>>>>  #include <linux/page_idle.h>
>>>>> +#include <linux/pagemap.h>
>>>>>  #include <linux/random.h>
>>>>> +#include <linux/rmap.h>
>>>>>  #include <linux/sched/mm.h>
>>>>>  #include <linux/sched/task.h>
>>>>>  #include <linux/slab.h>
>>>>> @@ -480,6 +482,11 @@ void kdamond_init_vm_regions(struct damon_ctx *ctx)
>>>>>  	}
>>>>>  }
>>>>>  
>>>>> +/* Do nothing.  Users should set the initial regions by themselves */
>>>>> +void kdamond_init_phys_regions(struct damon_ctx *ctx)
>>>>> +{
>>>>> +}
>>>>> +
>>>>>  static void damon_mkold(struct mm_struct *mm, unsigned long addr)
>>>>>  {
>>>>>  	pte_t *pte = NULL;
>>>>> @@ -611,6 +618,178 @@ unsigned int kdamond_check_vm_accesses(struct damon_ctx *ctx)
>>>>>  	return max_nr_accesses;
>>>>>  }
>>>>>  
>>>>> +/* access check functions for physical address based regions */
>>>>> +
>>>>> +/* This code is stollen from page_idle.c */
>>>>> +static struct page *damon_phys_get_page(unsigned long pfn)
>>>>> +{
>>>>> +	struct page *page;
>>>>> +	pg_data_t *pgdat;
>>>>> +
>>>>> +	if (!pfn_valid(pfn))
>>>>> +		return NULL;
>>>>> +
>>>>
>>>> Who provides these pfns? Can these be random pfns, supplied unchecked by
>>>> user space? Or are they at least mapped into some user space process?
>>>
>>> Your guess is right, users can give random physical address and that will be
>>> translated into pfn.
>>>
>>
>> Note the difference to idle tracking: "Idle page tracking only considers
>> user memory pages", this is very different to your use case. Note that
>> this is why there is no pfn_to_online_page() check in page idle code.
> 
> My use case is same to that of idle page.  I also ignore non-user pages.
> Actually, this function is for filtering of the non-user pages, which is simply
> stollen from the page_idle.

Okay, that is valuable information, I missed that. The comment in
page_idle.c is actually pretty valuable.

In both cases, user space can provide random physical address but you
will only care about user pages. Understood.

That turns things less dangerous. :)

>>>> IOW, do we need a pfn_to_online_page() to make sure the memmap even was
>>>> initialized?
>>>
>>> Thank you for pointing out this!  I will use it in the next spin.  Also, this
>>> code is stollen from page_idle_get_page().  Seems like it should also be
>>> modified to use it.  I will send the patch for it, either.
>>
>> pfn_to_online_page() will only succeed for system RAM pages, not
>> dax/pmem (ZONE_DEVICE). dax/pmem needs special care.
>>
>> I can spot that you are taking references to random struct pages. This
>> looks dangerous to me and might mess in complicated ways with page
>> migration/isolation/onlining/offlining etc. I am not sure if we want that.
> 
> AFAIU, page_idle users can also pass random pfns by randomly accessing the
> bitmap file.  Am I missing something?

I am definitely no expert on page idle tracking. If that is the case,
then we'll also need pfn_to_online_page() handling (and might have to
care about ZONE_DEVICE, not hard but needs some extra LOCs).

I am still not sure if grabbing references on theoretically isolated
pageblocks is okay, but that's just complicated stuff and as you state,
is already performed. At least I can read "With such an indicator of
user pages we can skip isolated pages". So isolated pages during page
migration are properly handled.


Instead of stealing, factor out, document, and reuse? That makes it
clearer that you are not inventing the wheel, and if we have to fix
something, we only have to fix at a single point.

-- 
Thanks,

David / dhildenb