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Subject: Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure

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():

	if (gfpflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
		alloc_flags |= ALLOC_CMA;

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.



David / dhildenb

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