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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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References: <20200604072611.11049-1-sjpark@amazon.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <4b5814d8-9626-e71c-9e5d-a4a61fcd12e8@redhat.com>
Date:   Thu, 4 Jun 2020 16:58:13 +0200
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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.

>>
>> 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.

-- 
Thanks,

David / dhildenb