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Subject: Re: [PATCH 6/6] mm: track gup pages with page->dma_pinned_* fields
To:     Balbir Singh <bsingharora@gmail.com>
CC:     Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        Christopher Lameter <cl@linux.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dan Williams <dan.j.williams@intel.com>,
        Jan Kara <jack@suse.cz>, <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-rdma <linux-rdma@vger.kernel.org>,
        <linux-fsdevel@vger.kernel.org>
References: <20181012060014.10242-1-jhubbard@nvidia.com>
 <20181012060014.10242-7-jhubbard@nvidia.com> <20181012110728.GL8537@350D>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <af901585-3f9a-da35-81a4-50ea341844c4@nvidia.com>
Date:   Fri, 12 Oct 2018 17:33:51 -0700
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On 10/12/18 4:07 AM, Balbir Singh wrote:
> On Thu, Oct 11, 2018 at 11:00:14PM -0700, john.hubbard@gmail.com wrote:
>> From: John Hubbard <jhubbard@nvidia.com>
[...]
>> +static int pin_page_for_dma(struct page *page)
>> +{
>> +	int ret = 0;
>> +	struct zone *zone;
>> +
>> +	page = compound_head(page);
>> +	zone = page_zone(page);
>> +
>> +	spin_lock(zone_gup_lock(zone));
>> +
>> +	if (PageDmaPinned(page)) {
>> +		/* Page was not on an LRU list, because it was DMA-pinned. */
>> +		VM_BUG_ON_PAGE(PageLRU(page), page);
>> +
>> +		atomic_inc(&page->dma_pinned_count);
>> +		goto unlock_out;
>> +	}
>> +
>> +	/*
>> +	 * Note that page->dma_pinned_flags is unioned with page->lru.
>> +	 * Therefore, the rules are: checking if any of the
>> +	 * PAGE_DMA_PINNED_FLAGS bits are set may be done while page->lru
>> +	 * is in use. However, setting those flags requires that
>> +	 * the page is both locked, and also, removed from the LRU.
>> +	 */
>> +	ret = isolate_lru_page(page);
>> +
> 
> isolate_lru_page() can be expensive and in terms of the overall locking order
> sounds like zone_gup_lock is higher in the hierarchy than the locks taken
> inside isolate_lru_page()
 
As for the expensive part, that is a concern. But I do think we need some lock
here. The hierarchy shouldn't be a problem, given that this is a new lock. But
I'm not sure how to make this better. In any case, I think it should work--note that
the zone_lru_lock, within isolate_lru_page(), is of similar use, and is held
for a similar duration, so...maybe not really a problem?


>> +	if (ret == 0) {
>> +		/* Avoid problems later, when freeing the page: */
>> +		ClearPageActive(page);
>> +		ClearPageUnevictable(page);
>> +
>> +		/* counteract isolate_lru_page's effects: */
>> +		put_page(page);
> 
> Can the page get reaped here? What's the expected page count?

Nope. The page_count is at least one, because get_user_pages() incremented it.

 
>> +
>> +		atomic_set(&page->dma_pinned_count, 1);
>> +		SetPageDmaPinned(page);
>> +	}
>> +
>> +unlock_out:
>> +	spin_unlock(zone_gup_lock(zone));
>> +
>> +	return ret;
>> +}
>> +
>>  static struct page *no_page_table(struct vm_area_struct *vma,
>>  		unsigned int flags)
>>  {
>> @@ -659,7 +704,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>>  		unsigned int gup_flags, struct page **pages,
>>  		struct vm_area_struct **vmas, int *nonblocking)
>>  {
>> -	long i = 0;
>> +	long i = 0, j;
>>  	int err = 0;
>>  	unsigned int page_mask;
>>  	struct vm_area_struct *vma = NULL;
>> @@ -764,6 +809,10 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>>  	} while (nr_pages);
>>  
>>  out:
>> +	if (pages)
>> +		for (j = 0; j < i; j++)
>> +			pin_page_for_dma(pages[j]);
>> +
> 
> Why does get_user_pages() unconditionally pin_page_for_dma?

That's the grand plan here: get_user_pages() now means "unconditionally pin the page for dma".
If you didn't want that, then either release it quickly (many callers do), or use a different
way of pinning or acquiring the page.

> 
>>  	return i ? i : err;
>>  }
>>  
>> @@ -1841,7 +1890,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
>>  			struct page **pages)
>>  {
>>  	unsigned long addr, len, end;
>> -	int nr = 0, ret = 0;
>> +	int nr = 0, ret = 0, i;
>>  
>>  	start &= PAGE_MASK;
>>  	addr = start;
>> @@ -1862,6 +1911,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
>>  		ret = nr;
>>  	}
>>  
>> +	for (i = 0; i < nr; i++)
>> +		pin_page_for_dma(pages[i]);
> 
> Why does get_user_pages_fast() unconditionally pin_page_for_dma?

All of the get_user_pages*() variants need to follow the same rules, so the same 
explanation as above, applies here also.

>> +
>>  	if (nr < nr_pages) {
>>  		/* Try to get the remaining pages with get_user_pages */
>>  		start += nr << PAGE_SHIFT;
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index e79cb59552d9..af9719756081 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2335,6 +2335,11 @@ static void lock_page_lru(struct page *page, int *isolated)
>>  	if (PageLRU(page)) {
>>  		struct lruvec *lruvec;
>>  
>> +		/* LRU and PageDmaPinned are mutually exclusive: they use the
>> +		 * same fields in struct page, but for different purposes.
>> +		 */
> 
> Comment style needs fixing
 
oops, thanks for spotting those, will fix.


-- 
thanks,
John Hubbard
NVIDIA