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
