Date:   Sun, 20 Nov 2016 23:51:48 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Jatin Kumar <jakumar@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM v13 16/18] mm/hmm/migrate: new memory migration helper for use with device memory

Jerome Glisse <jglisse@...hat.com> writes:

.....

>> > +
>> > +		*pfns = hmm_pfn_from_pfn(pfn) | HMM_PFN_MIGRATE | flags;
>> > +		*pfns |= write ? HMM_PFN_WRITE : 0;
>> > +		migrate->npages++;
>> > +		get_page(page);
>> > +
>> > +		if (!trylock_page(page)) {
>> > +			set_pte_at(mm, addr, ptep, pte);
>> > +		} else {
>> > +			pte_t swp_pte;
>> > +
>> > +			*pfns |= HMM_PFN_LOCKED;
>> > +
>> > +			entry = make_migration_entry(page, write);
>> > +			swp_pte = swp_entry_to_pte(entry);
>> > +			if (pte_soft_dirty(pte))
>> > +				swp_pte = pte_swp_mksoft_dirty(swp_pte);
>> > +			set_pte_at(mm, addr, ptep, swp_pte);
>> > +
>> > +			page_remove_rmap(page, false);
>> > +			put_page(page);
>> > +			pages++;
>> > +		}
>> 
>> If this is an optimization, can we get that as a seperate patch with
>> addtional comments. ? How does take a successful page lock implies it is
>> not a shared mapping ?
>
> It can be a share mapping and that's fine, migration only fail if page is
> pin.
>

In the previous mail you replied above trylock_page() usage is an
optimization for the usual case where the memory is only use in one
process and that no concurrent migration/memory event is happening. 

How did we know that it is only in use by one process. I got the part
that if we can lock, and since we lock the page early, it avoid
concurrent migration. But I am not sure about the use by one process
part. 


>
>> > +	}
>> > +
>> > +	arch_leave_lazy_mmu_mode();
>> > +	pte_unmap_unlock(ptep - 1, ptl);
>> > +
>> > +	/* Only flush the TLB if we actually modified any entries */
>> > +	if (pages)
>> > +		flush_tlb_range(walk->vma, start, end);
>> > +
>> > +	return 0;
>> > +}
>> 
>> 
>> So without the optimization the above function is suppose to raise the
>> refcount and collect all possible pfns tha we can migrate in the array ?
>
> Yes correct, this function collect all page we can migrate in the range.
>

.....

>
>> > +static void hmm_migrate_lock_and_isolate(struct hmm_migrate *migrate)
>> > +{
>> > +	unsigned long addr = migrate->start, i = 0;
>> > +	struct mm_struct *mm = migrate->vma->vm_mm;
>> > +	struct vm_area_struct *vma = migrate->vma;
>> > +	unsigned long restore = 0;
>> > +	bool allow_drain = true;
>> > +
>> > +	lru_add_drain();
>> > +
>> > +again:
>> > +	for (; addr < migrate->end; addr += PAGE_SIZE, i++) {
>> > +		struct page *page = hmm_pfn_to_page(migrate->pfns[i]);
>> > +
>> > +		if (!page)
>> > +			continue;
>> > +
>> > +		if (!(migrate->pfns[i] & HMM_PFN_LOCKED)) {
>> > +			lock_page(page);
>> > +			migrate->pfns[i] |= HMM_PFN_LOCKED;
>> > +		}
>> 
>> What does taking a page_lock protect against ? Can we document that ?
>
> This usual page migration process like existing code, page lock protect against
> anyone trying to map the page inside another process or at different address. It
> also block few fs operations. I don't think there is a comprehensive list anywhere
> but i can try to make one.


I was comparing it against the trylock_page() usage above. But I guess
documenting the page lock can be another patch. 


>
>> > +
>> > +		/* ZONE_DEVICE page are not on LRU */
>> > +		if (is_zone_device_page(page))
>> > +			goto check;
>> > +
>> > +		if (!PageLRU(page) && allow_drain) {
>> > +			/* Drain CPU's pagevec so page can be isolated */
>> > +			lru_add_drain_all();
>> > +			allow_drain = false;
>> > +			goto again;
>> > +		}
>> > +
>> > +		if (isolate_lru_page(page)) {
>> > +			migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
>> > +			migrate->npages--;
>> > +			put_page(page);
>> > +			restore++;
>> > +		} else
>> > +			/* Drop the reference we took in collect */
>> > +			put_page(page);
>> > +
>> > +check:
>> > +		if (!hmm_migrate_page_check(page, 1)) {
>> > +			migrate->pfns[i] &= ~HMM_PFN_MIGRATE;
>> > +			migrate->npages--;
>> > +			restore++;
>> > +		}
>> > +	}
>> > +
> 

.....

>> > +		}
>> > +		pte_unmap_unlock(ptep - 1, ptl);
>> > +
>> > +		addr = restart;
>> > +		i = (addr - migrate->start) >> PAGE_SHIFT;
>> > +		for (; addr < next && restore; addr += PAGE_SHIFT, i++) {
>> > +			page = hmm_pfn_to_page(migrate->pfns[i]);
>> > +			if (!page || (migrate->pfns[i] & HMM_PFN_MIGRATE))
>> > +				continue;
>> > +
>> > +			migrate->pfns[i] = 0;
>> > +			unlock_page(page);
>> > +			restore--;
>> > +
>> > +			if (is_zone_device_page(page)) {
>> > +				put_page(page);
>> > +				continue;
>> > +			}
>> > +
>> > +			putback_lru_page(page);
>> > +		}
>> > +
>> > +		if (!restore)
>> > +			break;
>> > +	}
>> 
>> 
>> All the above restore won't be needed if we didn't do that migration
>> entry setup in the first function right ? We just need to drop the
>> refcount for pages that we failed to isolated ? No need to walk the page
>> table etc ?
>
> Well the migration entry setup is important so that no concurrent migration
> can race with each other, the one that set the migration entry first is the
> one that win in respect of migration. Also the CPU page table entry need to
> be clear so that page content is stable and DMA copy does not miss any data
> left over in some cache.

This is the part i am still tryint to understand. 
hmm_collect_walk_pmd(), did migration entry setup only in one process
page table. So how can it prevent concurrent migration because one could
initiate a migration using the va/mapping of another process.

Isn't that page lock that is prevent concurrent migration ?

........

> 
>> Why are we walking the page table multiple times ? Is it that after
>> alloc_copy the content of migrate->pfns pfn array is now the new pfns ?
>> It is confusing that each of these functions walk one page table
>> multiple times (even when page can be shared). I was expecting us to
>> walk the page table once to collect the pfns/pages and then use that
>> in rest of the calls. Any specific reason you choose to implement it
>> this way ?
>
> Well you need to know the source and destination page, so either i have
> 2 arrays one for source page and one for destination pages and then i do
> not need to walk page table multiple time. But needing 2 arrays might be
> problematic as here we want to migrate reasonable chunk ie few megabyte
> hence there is a need for vmalloc.
>
> My advice to device driver was to pre-allocate this array once (maybe
> preallocate couple of them). If you really prefer avoiding walking the
> CPU page table over and over then i can switch to 2 arrays solutions.
>

Having two array makes it easy to follow the code. But otherwise I guess
documenting the above usage of page table above the function will also
help.

.....

>> IMHO If we can get each of the above functions documented properly it will
>> help with code review. Also if we can avoid that multiple page table
>> walk, it will make it closer to the existing migration logic.
>> 
>
> What kind of documentation are you looking for ? I thought the high level overview
> was enough as none of the function do anything out of the ordinary. Do you want
> more inline documation ? Or a more verbose highlevel overview ?


Inline documentation for functions will be useful. Also if you can split
the hmm_collect_walk_pmd() optimization we discussed above into a
separate patch I guess this will be lot easy to follow.

I still haven't understood why we setup that migration entry early and
that too only on one process page table. If we can explain that as a
separate patch may be it will much easy to follow.

-aneesh

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