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

[Balbir Singh]

On Fri, Oct 12, 2018 at 05:15:51PM -0700, John Hubbard wrote:
> 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);
> }
> 
>

Sorry, I've been distracted with other things

This looks better, do we still need the INIT_LIST_HEAD?

Balbir Singh.
 
> 
> -- 
> thanks,
> John Hubbard
> NVIDIA