Re: [virtio-dev] Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure

Nitesh Narayan Lal
2019-08-14T12:49:45Z

On 8/14/19 3:07 AM, David Hildenbrand wrote:
> On 14.08.19 01:14, Alexander Duyck wrote:
>> On Tue, Aug 13, 2019 at 3:34 AM David Hildenbrand <david@redhat.com> wrote:
>>>>>>> +static int process_free_page(struct page *page,
>>>>>>> + struct page_reporting_config *phconf, int count)
>>>>>>> +{
>>>>>>> + int mt, order, ret = 0;
>>>>>>> +
>>>>>>> + mt = get_pageblock_migratetype(page);
>>>>>>> + order = page_private(page);
>>>>>>> + ret = __isolate_free_page(page, order);
>>>>>>> +
>>>>> I just started looking into the wonderful world of
>>>>> isolation/compaction/migration.
>>>>>
>>>>> I don't think saving/restoring the migratetype is correct here. AFAIK,
>>>>> MOVABLE/UNMOVABLE/RECLAIMABLE is just a hint, doesn't mean that e.g.,
>>>>> movable pages and up in UNMOVABLE or ordinary kernel allocations on
>>>>> MOVABLE. So that shouldn't be an issue - I guess.
>>>>>
>>>>> 1. You should never allocate something that is no
>>>>> MOVABLE/UNMOVABLE/RECLAIMABLE. Especially not, if you have ISOLATE or
>>>>> CMA here. There should at least be a !is_migrate_isolate_page() check
>>>>> somewhere
>>>>>
>>>>> 2. set_migratetype_isolate() takes the zone lock, so to avoid racing
>>>>> with isolation code, you have to hold the zone lock. Your code seems to
>>>>> do that, so at least you cannot race against isolation.
>>>>>
>>>>> 3. You could end up temporarily allocating something in the
>>>>> ZONE_MOVABLE. The pages you allocate are, however, not movable. There
>>>>> would have to be a way to make alloc_contig_range()/offlining code
>>>>> properly wait until the pages have been processed. Not sure about the
>>>>> real implications, though - too many details in the code (I wonder if
>>>>> Alex' series has a way of dealing with that)
>>>>>
>>>>> When you restore the migratetype, you could suddenly overwrite e.g.,
>>>>> ISOLATE, which feels wrong.
>>>>
>>>> I was triggering an occasional CPU stall bug earlier, with saving and restoring
>>>> the migratetype I was able to fix it.
>>>> But I will further look into this to figure out if it is really required.
>>>>
>>> You should especially look into handling isolated/cma pages. Maybe that
>>> was the original issue. Alex seems to have added that in his latest
>>> series (skipping isolated/cma pageblocks completely) as well.
>> So as far as skipping isolated pageblocks, I get the reason for
>> skipping isolated, but why would we need to skip CMA? I had made the
>> change I did based on comments you had made earlier. But while working
>> on some of the changes to address isolation better and looking over
>> several spots in the code it seems like CMA is already being used as
>> an allocation fallback for MIGRATE_MOVABLE. If that is the case
>> wouldn't it make sense to allow pulling pages and reporting them while
>> they are in the free_list?
> I was assuming that CMA is also to be skipped because "static int
> fallbacks[MIGRATE_TYPES][4]" in mm/page_alloc.c doesn't handle CMA at
> all, meaning we should never fallback to CMA or from CMA to another type
> - at least when stealing pages from another migratetype. So it smells
> like MIGRATE_CMA is static -> the area is marked once and will never be
> converted to something else (except MIGRATE_ISOLATE temporarily).
>
> I assume you are talking about gfp_to_alloc_flags()/prepare_alloc_pages():

I am also trying to look into this to get more understanding of it.
Another thing which I am looking into right now is the difference between
get/set_pcppage_migratetype() and ge/set_pageblock_migratetype().
To an extent, I do understand what is the benefit of using
get/set_pcppage_migratetype() by reading the comments. However, I am not sure
how it gets along with MIGRATE_CMA.
Hopefully, I will have an understanding of it soon.

> #ifdef CONFIG_CMA
> if (gfpflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> alloc_flags |= ALLOC_CMA;
> #endif
>
> Yeah, this looks like MOVABLE allocations can fallback to CMA
> pageblocks. And from what I read, "CMA may use its own migratetype
> (MIGRATE_CMA) which behaves similarly to ZONE_MOVABLE but can be put in
> arbitrary places."
>
> So I think you are right, it could be that it is safe to temporarily
> pull out CMA pages (in contrast to isolated pages) - assuming it is fine
> to have temporary unmovable allocations on them (different discussion).
>
> (I am learning about the details as we discuss :) )
>
> The important part would then be to never allocate from the isolated
> pageblocks and to never overwrite MIGRATE_ISOLATE.

Agreed. I think I should just avoid isolating pages with migratetype
MIGRATE_ISOLATE.
Adding a check with is_migrate_isolate_page() before isolating the page should
do it.

>
--
Thanks
Nitesh