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

David Hildenbrand
2020-06-04T14:58:37Z

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