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Subject[PATCH] mm/highmem.c
Hi,

mm/highmem.c code tweaks against 2.4.9-pre4;

o In flush_all_zero_pkmaps(), the atomic fetch-and-clear pte op;
ptep_get_and_clear(), is overkill - the holding of the
kmap_lock is sufficient. Use a lighter pte_clear().

o In kunmap_high(), the common case is for the count to be 1 which
causes a call to wake_up(). It is possible to be lighter, as
the kmap_lock is already held and additions to the wait-queue occur
under this lock. Simply test for an "active" wait-queue under
the kmap_lock, and call wake_up() after dropping the lock.

o As I've previously pointed out, copy_from_high_bh() cannot
be called from an interrupt context, so doesn't need to mask
interrupts.

Thanks,
Mark


diff -ur -X dontdiff linux-2.4.9-pre4/mm/highmem.c high-2.4.9-pre4/mm/highmem.c
--- linux-2.4.9-pre4/mm/highmem.c Tue Aug 7 08:18:00 2001
+++ high-2.4.9-pre4/mm/highmem.c Thu Aug 16 16:21:55 2001
@@ -46,7 +46,7 @@

for (i = 0; i < LAST_PKMAP; i++) {
struct page *page;
- pte_t pte;
+
/*
* zero means we don't have anything to do,
* >1 means that it is still in use. Only
@@ -56,10 +56,21 @@
if (pkmap_count[i] != 1)
continue;
pkmap_count[i] = 0;
- pte = ptep_get_and_clear(pkmap_page_table+i);
- if (pte_none(pte))
+
+ /* sanity check */
+ if (pte_none(pkmap_page_table[i]))
BUG();
- page = pte_page(pte);
+
+ /*
+ * Don't need an atomic fetch-and-clear op here;
+ * no-one has the page mapped, and cannot get at
+ * its virtual address (and hence PTE) without first
+ * getting the kmap_lock (which is held here).
+ * So no dangers, even with speculative execution.
+ */
+ page = pte_page(pkmap_page_table[i]);
+ pte_clear(&pkmap_page_table[i]);
+
page->virtual = NULL;
}
flush_tlb_all();
@@ -139,6 +150,7 @@
{
unsigned long vaddr;
unsigned long nr;
+ int need_wakeup;

spin_lock(&kmap_lock);
vaddr = (unsigned long) page->virtual;
@@ -150,13 +162,28 @@
* A count must never go down to zero
* without a TLB flush!
*/
+ need_wakeup = 0;
switch (--pkmap_count[nr]) {
case 0:
BUG();
case 1:
- wake_up(&pkmap_map_wait);
+ /*
+ * Avoid an unnecessary wake_up() function call.
+ * The common case is pkmap_count[] == 1, but
+ * no waiters.
+ * The tasks queued in the wait-queue are guarded
+ * by both the lock in the wait-queue-head and by
+ * the kmap_lock. As the kmap_lock is held here,
+ * no need for the wait-queue-head's lock. Simply
+ * test if the queue is empty.
+ */
+ need_wakeup = waitqueue_active(&pkmap_map_wait);
}
spin_unlock(&kmap_lock);
+
+ /* do wake-up, if needed, race-free outside of the spin lock */
+ if (need_wakeup)
+ wake_up(&pkmap_map_wait);
}

#define POOL_SIZE 32
@@ -182,20 +209,12 @@
{
struct page *p_from;
char *vfrom;
- unsigned long flags;

p_from = from->b_page;

- /*
- * Since this can be executed from IRQ context, reentrance
- * on the same CPU must be avoided:
- */
- __save_flags(flags);
- __cli();
vfrom = kmap_atomic(p_from, KM_BOUNCE_WRITE);
memcpy(to->b_data, vfrom + bh_offset(from), to->b_size);
kunmap_atomic(vfrom, KM_BOUNCE_WRITE);
- __restore_flags(flags);
}

static inline void copy_to_high_bh_irq (struct buffer_head *to,
-
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