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SubjectRe: [RFC] atomic highmem kmap page pinning
On Wed, 04 Mar 2009 00:58:13 -0500 (EST)
Nicolas Pitre <nico@cam.org> wrote:

> I've implemented highmem for ARM. Yes, some ARM machines do have lots
> of memory...
>
> The problem is that most ARM machines have a non IO coherent cache,
> meaning that the dma_map_* set of functions must clean and/or invalidate
> the affected memory manually. And because the majority of those
> machines have a VIVT cache, the cache maintenance operations must be
> performed using virtual addresses.
>
> In dma_map_page(), an highmem pages could still be mapped and cached
> even after kunmap() was called on it. As long as highmem pages are
> mapped, page_address(page) is non null and we can use that to
> synchronize the cache.
> It is unlikely but still possible for kmap() to race and recycle the
> obtained virtual address above, and use it for another page though. In
> that case, the new mapping could end up with dirty cache lines for
> another page, and the unsuspecting cache invalidation loop in
> dma_map_page() won't notice resulting in data loss. Hence the need for
> some kind of kmap page pinning which can be used in any context,
> including IRQ context.
>
> This is a RFC patch implementing the necessary part in the core code, as
> suggested by RMK. Please comment.

I am not sure if i understand your concern totally.
I can understand it can be recycled. but Why is it racing ?
Now, kmap semantic is that it can't be called in interrupt context.

As far as I understand, To make irq_disable to prevent this problem is rather big cost.

I think it would be better to make page_address can return null in that case
where pkmap_count is less than one or it's not previous page mapping.


> diff --git a/mm/highmem.c b/mm/highmem.c
> index b36b83b..548ca77 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -113,9 +113,9 @@ static void flush_all_zero_pkmaps(void)
> */
> void kmap_flush_unused(void)
> {
> - spin_lock(&kmap_lock);
> + spin_lock_irq(&kmap_lock);
> flush_all_zero_pkmaps();
> - spin_unlock(&kmap_lock);
> + spin_unlock_irq(&kmap_lock);
> }
>
> static inline unsigned long map_new_virtual(struct page *page)
> @@ -145,10 +145,10 @@ start:
>
> __set_current_state(TASK_UNINTERRUPTIBLE);
> add_wait_queue(&pkmap_map_wait, &wait);
> - spin_unlock(&kmap_lock);
> + spin_unlock_irq(&kmap_lock);
> schedule();
> remove_wait_queue(&pkmap_map_wait, &wait);
> - spin_lock(&kmap_lock);
> + spin_lock_irq(&kmap_lock);
>
> /* Somebody else might have mapped it while we slept */
> if (page_address(page))
> @@ -184,19 +184,43 @@ void *kmap_high(struct page *page)
> * For highmem pages, we can't trust "virtual" until
> * after we have the lock.
> */
> - spin_lock(&kmap_lock);
> + spin_lock_irq(&kmap_lock);
> vaddr = (unsigned long)page_address(page);
> if (!vaddr)
> vaddr = map_new_virtual(page);
> pkmap_count[PKMAP_NR(vaddr)]++;
> BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 2);
> - spin_unlock(&kmap_lock);
> + spin_unlock_irq(&kmap_lock);
> return (void*) vaddr;
> }
>
> EXPORT_SYMBOL(kmap_high);
>
> /**
> + * kmap_high_get - pin a highmem page into memory
> + * @page: &struct page to pin
> + *
> + * Returns the page's current virtual memory address, or NULL if no mapping
> + * exists. When and only when a non null address is returned then a
> + * matching call to kunmap_high() is necessary.
> + *
> + * This can be called from interrupt context.
> + */
> +void *kmap_high_get(struct page *page)
> +{
> + unsigned long vaddr, flags;
> +
> + spin_lock_irqsave(&kmap_lock, flags);
> + vaddr = (unsigned long)page_address(page);
> + if (vaddr) {
> + BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 1);
> + pkmap_count[PKMAP_NR(vaddr)]++;
> + }
> + spin_unlock_irqrestore(&kmap_lock, flags);
> + return (void*) vaddr;
> +}
> +
> +/**
> * kunmap_high - map a highmem page into memory
> * @page: &struct page to unmap
> */
> @@ -204,9 +228,10 @@ void kunmap_high(struct page *page)
> {
> unsigned long vaddr;
> unsigned long nr;
> + unsigned long flags;
> int need_wakeup;
>
> - spin_lock(&kmap_lock);
> + spin_lock_irqsave(&kmap_lock, flags);
> vaddr = (unsigned long)page_address(page);
> BUG_ON(!vaddr);
> nr = PKMAP_NR(vaddr);
> @@ -232,7 +257,7 @@ void kunmap_high(struct page *page)
> */
> need_wakeup = waitqueue_active(&pkmap_map_wait);
> }
> - spin_unlock(&kmap_lock);
> + spin_unlock_irqrestore(&kmap_lock, flags);
>
> /* do wake-up, if needed, race-free outside of the spin lock */
> if (need_wakeup)
>
>
> Nicolas
> --
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--
Kinds Regards
Minchan Kim


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