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Subject: Re: [RFC v2 7/9] mm/damon: Implement callbacks for physical memory
 monitoring
To:     SeongJae Park <sjpark@amazon.com>
Cc:     akpm@linux-foundation.org, SeongJae Park <sjpark@amazon.de>,
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From:   David Hildenbrand <david@redhat.com>
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Message-ID: <b0cce3d0-07c1-0468-f64a-21cefdb74a5d@redhat.com>
Date:   Thu, 4 Jun 2020 18:01:28 +0200
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On 04.06.20 17:51, SeongJae Park wrote:
> On Thu, 4 Jun 2020 17:39:49 +0200 David Hildenbrand <david@redhat.com> wrote:
> 
>> On 04.06.20 17:23, SeongJae Park wrote:
>>> On Thu, 4 Jun 2020 16:58:13 +0200 David Hildenbrand <david@redhat.com> wrote:
>>>
>>>> On 04.06.20 09:26, SeongJae Park wrote:
>>>>> On Wed, 3 Jun 2020 18:09:21 +0200 David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>>> On 03.06.20 16:11, SeongJae Park wrote:
>>>>>>> From: SeongJae Park <sjpark@amazon.de>
>>>>>>>
>>>>>>> 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@amazon.de>
>>>>>>> ---
>>>>>>>  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. :)
> 
> Glad to hear this.  I will refine this point in the next spin! :)

Perfect, when I read "physical address space", I was assuming you would
magically track access to e.g., memmap, page tables and stuff like that.
And I wondered how you would do that :D

BTW now that I realized that as we only care about LRU pages,
page_to_online_page() is sufficient, no need to worry about dax/pmem.

-- 
Thanks,

David / dhildenb