linux-kernel - Re: [PATCH v2 6/6] mm: page_mkclean, ttu: handle pinned pages

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Date:   Mon, 2 Jul 2018 14:07:44 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>, <john.hubbard@...il.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 6/6] mm: page_mkclean, ttu: handle pinned pages

On 07/02/2018 03:15 AM, Jan Kara wrote:
> On Sun 01-07-18 17:56:54, john.hubbard@...il.com wrote:
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 9d142b9b86dc..c4bc8d216746 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -931,6 +931,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
>>  	int kill = 1, forcekill;
>>  	struct page *hpage = *hpagep;
>>  	bool mlocked = PageMlocked(hpage);
>> +	bool skip_pinned_pages = false;
> 
> I'm not sure we can afford to wait for page pins when handling page
> poisoning. In an ideal world we should but... But I guess this is for
> someone understanding memory poisoning better to judge.


OK, then until I hear otherwise, in the next version I'll set 
skipped_pinned_pages = true here, based on the idea that it's probably
better to be sure we don't hang while trying to remove a bad page. It's
hard to achieve perfection in the presence of a memory failure.

> 
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 6db729dc4c50..c137c43eb2ad 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -879,6 +879,26 @@ int page_referenced(struct page *page,
>>  	return pra.referenced;
>>  }
>>  
>> +/* Must be called with pinned_dma_lock held. */
>> +static void wait_for_dma_pinned_to_clear(struct page *page)
>> +{
>> +	struct zone *zone = page_zone(page);
>> +
>> +	while (PageDmaPinnedFlags(page)) {
>> +		spin_unlock(zone_gup_lock(zone));
>> +
>> +		schedule();
>> +
>> +		spin_lock(zone_gup_lock(zone));
>> +	}
>> +}
> 
> Ouch, we definitely need something better here. Either reuse the
> page_waitqueue() mechanism or create at least a global wait queue for this
> (I don't expect too much contention on the waitqueue and even if there
> eventually is, we can switch to page_waitqueue() when we find it).  But
> this is a no-go...

Yes, no problem. At one point I had a separate bit waiting queue, which was
only a few lines of code to do, but I dropped it because I thought that maybe 
it was overkill. I'll put it back in.

> 
>> +
>> +struct page_mkclean_info {
>> +	int cleaned;
>> +	int skipped;
>> +	bool skip_pinned_pages;
>> +};
>> +
>>  static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
>>  			    unsigned long address, void *arg)
>>  {
>> @@ -889,7 +909,24 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
>>  		.flags = PVMW_SYNC,
>>  	};
>>  	unsigned long start = address, end;
>> -	int *cleaned = arg;
>> +	struct page_mkclean_info *mki = (struct page_mkclean_info *)arg;
>> +	bool is_dma_pinned;
>> +	struct zone *zone = page_zone(page);
>> +
>> +	/* Serialize with get_user_pages: */
>> +	spin_lock(zone_gup_lock(zone));
>> +	is_dma_pinned = PageDmaPinned(page);
> 
> Hum, why do you do this for each page table this is mapped in? Also the
> locking is IMHO going to hurt a lot and we need to avoid it.
> 
> What I think needs to happen is that in page_mkclean(), after you've
> cleared all the page tables, you check PageDmaPinned() and wait if needed.
> Page cannot be faulted in again as we hold page lock and so races with
> concurrent GUP are fairly limited. So with some careful ordering & memory
> barriers you should be able to get away without any locking. Ditto for the
> unmap path...
> 

I guess I was thinking about this backwards. It would work much better if
we go ahead and write protect or unmap first, let things drain, and wait later.
Very nice!


thanks,
-- 
John Hubbard
NVIDIA