Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count

[John Hubbard]

On 10/12/18 3:56 AM, Balbir Singh wrote:
> On Thu, Oct 11, 2018 at 11:00:12PM -0700, john.hubbard@xxxxxxxxx wrote:
>> From: John Hubbard <jhubbard@xxxxxxxxxx>
[...]
>> + * Because page->dma_pinned_flags is unioned with page->lru, any page that
>> + * uses these flags must NOT be on an LRU. That's partly enforced by
>> + * ClearPageDmaPinned, which gives the page back to LRU.
>> + *
>> + * PageDmaPinned also corresponds to PageTail (the 0th bit in the first union
>> + * of struct page), and this flag is checked without knowing whether it is a
>> + * tail page or a PageDmaPinned page. Therefore, start the flags at bit 1 (0x2),
>> + * rather than bit 0.
>> + */
>> +#define PAGE_DMA_PINNED		0x2
>> +#define PAGE_DMA_PINNED_FLAGS	(PAGE_DMA_PINNED)
>> +
> 
> This is really subtle, additional changes to compound_head will need to coordinate
> with these flags? Also doesn't this bit need to be unique across all structs in
> the union? I guess that is guaranteed by the fact that page == compound_head(page)
> as per your assertion, but I've forgotten why that is true. Could you please
> add some commentary on that
> 

Yes, agreed. I've rewritten and augmented that comment block, plus removed the 
PAGE_DMA_PINNED_FLAGS (there are no more bits available, so it's just misleading 
to even have it). So now it looks like this:

/*
 * Because page->dma_pinned_flags is unioned with page->lru, any page that
 * uses these flags must NOT be on an LRU. That's partly enforced by
 * ClearPageDmaPinned, which gives the page back to LRU.
 *
 * PageDmaPinned is checked without knowing whether it is a tail page or a
 * PageDmaPinned page. For that reason, PageDmaPinned avoids PageTail (the 0th
 * bit in the first union of struct page), and instead uses bit 1 (0x2),
 * rather than bit 0.
 *
 * PageDmaPinned can only be used if no other systems are using the same bit
 * across the first struct page union. In this regard, it is similar to
 * PageTail, and in fact, because of PageTail's constraint that bit 0 be left
 * alone, bit 1 is also left alone so far: other union elements (ignoring tail
 * pages) put pointers there, and pointer alignment leaves the lower two bits
 * available.
 *
 * So, constraints include:
 *
 *     -- Only use PageDmaPinned on non-tail pages.
 *     -- Remove the page from any LRU list first.
 */
#define PAGE_DMA_PINNED		0x2

/*
 * Because these flags are read outside of a lock, ensure visibility between
 * different threads, by using READ|WRITE_ONCE.
 */
static __always_inline int PageDmaPinned(struct page *page)
{
	VM_BUG_ON(page != compound_head(page));
	return (READ_ONCE(page->dma_pinned_flags) & PAGE_DMA_PINNED) != 0;
}

[...]
>> +static __always_inline void SetPageDmaPinned(struct page *page)
>> +{
>> +	VM_BUG_ON(page != compound_head(page));
> 
> VM_BUG_ON(!list_empty(&page->lru))


There is only one place where we set this flag, and that is when (in patch 6/6)
transitioning from a page that might (or might not) have been
on an LRU. In that case, the calling code has already corrupted page->lru, by
writing to page->dma_pinned_count, which is unions with page->lru:

		atomic_set(&page->dma_pinned_count, 1);
		SetPageDmaPinned(page);

...so it would be inappropriate to call a list function, such as 
list_empty(), on that field.  Let's just leave it as-is.


> 
>> +	WRITE_ONCE(page->dma_pinned_flags, PAGE_DMA_PINNED);
>> +}
>> +
>> +static __always_inline void ClearPageDmaPinned(struct page *page)
>> +{
>> +	VM_BUG_ON(page != compound_head(page));
>> +	VM_BUG_ON_PAGE(!PageDmaPinnedFlags(page), page);
>> +
>> +	/* This does a WRITE_ONCE to the lru.next, which is also the
>> +	 * page->dma_pinned_flags field. So in addition to restoring page->lru,
>> +	 * this provides visibility to other threads.
>> +	 */
>> +	INIT_LIST_HEAD(&page->lru);
> 
> This assumes certain things about list_head, why not use the correct
> initialization bits.
> 

Yes, OK, changed to:

static __always_inline void ClearPageDmaPinned(struct page *page)
{
	VM_BUG_ON(page != compound_head(page));
	VM_BUG_ON_PAGE(!PageDmaPinned(page), page);

	/* Provide visibility to other threads: */
	WRITE_ONCE(page->dma_pinned_flags, 0);

	/*
	 * Safety precaution: restore the list head, before possibly returning
	 * the page to other subsystems.
	 */
	INIT_LIST_HEAD(&page->lru);
}



-- 
thanks,
John Hubbard
NVIDIA