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SubjectRe: [RFC] atomic highmem kmap page pinning
On Thu, 5 Mar 2009, Minchan Kim wrote:

> On Wed, 04 Mar 2009 21:37:43 -0500 (EST)
> Nicolas Pitre <> wrote:
> > My assertion is that the cost is negligible. This is why I'm asking you
> > why you think this is a big cost.
> Of course, I am not sure whether it's big cost or not.
> But I thought it already is used in many fs, driver.
> so, whether it's big cost depends on workload type .
> However, This patch is needed for VIVT and no coherent cache.
> Is right ?
> If it is right, it will add unnessary overhead in other architecture
> which don't have this problem.
> I think it's not desirable although it is small cost.
> If we have a other method which avoids unnessary overhead, It would be better.
> Unfortunately, I don't have any way to solve this, now.

OK. What about this patch then:

From c4db60c3a2395476331b62e08cf1f64fc9af8d54 Mon Sep 17 00:00:00 2001
From: Nicolas Pitre <>
Date: Wed, 4 Mar 2009 22:49:41 -0500
Subject: [PATCH] atomic highmem kmap page pinning

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 before DMA occurs. And because the majority of those
machines have a VIVT cache, the cache maintenance operations must be
performed using virtual

When a highmem page is kunmap'd, its mapping (and cache) remains in place
in case it is kmap'd again. However if dma_map_page() is then called with
such a page, some cache maintenance on the remaining mapping must be
performed. In that case, 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
virtual address obtained above, and use it for another page before some
on-going cache invalidation loop in dma_map_page() is done. 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() might
simply discard those dirty cache lines resulting in data loss.

For example, let's consider this sequence of events:

- dma_map_page(..., DMA_FROM_DEVICE) is called on a highmem page.

--> - vaddr = page_address(page) is non null. In this case
it is likely that the page has valid cache lines
associated with vaddr. Remember that the cache is VIVT.

--> for (i = vaddr; i < vaddr + PAGE_SIZE; i += 32)

*** preemption occurs in the middle of the loop above ***

- kmap_high() is called for a different page.

--> - last_pkmap_nr wraps to zero and flush_all_zero_pkmaps()
is called. The pkmap_count value for the page passed
to dma_map_page() above happens to be 1, so the page
is unmapped. But prior to that, flush_cache_kmaps()
cleared the cache for it. So far so good.

- A fresh pkmap entry is assigned for this kmap request.
The Murphy law says this pkmap entry will eventually
happen to use the same vaddr as the one which used to
belong to the other page being processed by
dma_map_page() in the preempted thread above.

- The kmap_high() caller start dirtying the cache using the
just assigned virtual mapping for its page.

*** the first thread is rescheduled ***

- The for(...) loop is resumed, but now cached
data belonging to a different physical page is
being discarded !

And this is not only a preemption issue as ARM can be SMP as well,
making the above scenario just as likely. Hence the need for some kind
of pkmap page pinning which can be used in any context, primarily for
the benefit of dma_map_page() on ARM.

This provides the necessary interface to cope with the above issue if
ARCH_NEEDS_KMAP_HIGH_GET is defined, otherwise the resulting code is

Signed-off-by: Nicolas Pitre <>

diff --git a/mm/highmem.c b/mm/highmem.c
index b36b83b..cc61399 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -67,6 +67,25 @@ pte_t * pkmap_page_table;

static DECLARE_WAIT_QUEUE_HEAD(pkmap_map_wait);

+ * Most architectures have no use for kmap_high_get(), so let's abstract
+ * the disabling of IRQ out of the locking in that case to save on a
+ * potential useless overhead.
+ */
+#define spin_lock_kmap() spin_lock_irq(&kmap_lock)
+#define spin_unlock_kmap() spin_unlock_irq(&kmap_lock)
+#define spin_lock_kmap_any(flags) spin_lock_irqsave(&kmap_lock, flags)
+#define spin_unlock_kmap_any(flags) spin_unlock_irqrestore(&kmap_lock, flags)
+#define spin_lock_kmap() spin_lock(&kmap_lock)
+#define spin_unlock_kmap() spin_unlock(&kmap_lock)
+#define spin_lock_kmap_any(flags) \
+ do { spin_lock(&kmap_lock); (void)(flags); } while (0)
+#define spin_unlock_kmap_any(flags) \
+ do { spin_unlock(&kmap_lock); (void)(flags); } while (0)
static void flush_all_zero_pkmaps(void)
int i;
@@ -113,9 +132,9 @@ static void flush_all_zero_pkmaps(void)
void kmap_flush_unused(void)
- spin_lock(&kmap_lock);
+ spin_lock_kmap();
- spin_unlock(&kmap_lock);
+ spin_unlock_kmap();

static inline unsigned long map_new_virtual(struct page *page)
@@ -145,10 +164,10 @@ start:

add_wait_queue(&pkmap_map_wait, &wait);
- spin_unlock(&kmap_lock);
+ spin_unlock_kmap();
remove_wait_queue(&pkmap_map_wait, &wait);
- spin_lock(&kmap_lock);
+ spin_lock_kmap();

/* Somebody else might have mapped it while we slept */
if (page_address(page))
@@ -184,29 +203,59 @@ 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_kmap();
vaddr = (unsigned long)page_address(page);
if (!vaddr)
vaddr = map_new_virtual(page);
BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 2);
- spin_unlock(&kmap_lock);
+ spin_unlock_kmap();
return (void*) vaddr;


+ * 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 any context.
+ */
+void *kmap_high_get(struct page *page)
+ unsigned long vaddr, flags;
+ spin_lock_kmap_any(flags);
+ vaddr = (unsigned long)page_address(page);
+ if (vaddr) {
+ BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 1);
+ pkmap_count[PKMAP_NR(vaddr)]++;
+ }
+ spin_unlock_kmap_any(flags);
+ return (void*) vaddr;
* kunmap_high - map a highmem page into memory
* @page: &struct page to unmap
+ *
+ * If ARCH_NEEDS_KMAP_HIGH_GET is not defined then this may be called
+ * only from user context.
void kunmap_high(struct page *page)
unsigned long vaddr;
unsigned long nr;
+ unsigned long flags;
int need_wakeup;

- spin_lock(&kmap_lock);
+ spin_lock_kmap_any(flags);
vaddr = (unsigned long)page_address(page);
nr = PKMAP_NR(vaddr);
@@ -232,7 +281,7 @@ void kunmap_high(struct page *page)
need_wakeup = waitqueue_active(&pkmap_map_wait);
- spin_unlock(&kmap_lock);
+ spin_unlock_kmap_any(flags);

/* do wake-up, if needed, race-free outside of the spin lock */
if (need_wakeup)

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