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SubjectRe: [RFC] Direct Sockets Support??
Venkateshr@ami.com said:
> > But in the case of an application which fits in main memory, and
> > has been running for a while (so all pages are present and
> > dirty), all you'd really have to do is verify the page tables are
> > in the proper state and skip the TLB flush, right?
>
> We really cannot assume this. There are two cases
> a. when a user app wants to receive some data, it allocates
> memory(using malloc) and waits for the hw to do zero-copy read. The kernel
> does not allocate physical page frames for the entire memory region
> allocated. We need to lock the memory (and locking is expensive due to
> costly TLB flushes) to do this
>
> b. when a user app wants to send data, he fills the buffer
> and waits for the hw to transmit data, but under heavy physical memory
> pressure, the swapper might swap the pages we want to transmit. So we need
> to lock the memory to be 100% sure.

You're right, of course. But I suspect that the fast path of
re-locking memory which is happily in core will go much faster
by removing the multi-processor TLB purge. And it can't hurt,
unless I'm missing something.

-- Pete

--- linux-2.4.4-stock/mm/mlock.c Tue May 8 17:26:34 2001
+++ linux/mm/mlock.c Tue May 8 17:24:13 2001
@@ -114,6 +114,10 @@
return 0;
}

+/* implemented in mm/memory.c */
+extern int mlock_make_pages_present(struct vm_area_struct *vma,
+ unsigned long addr, unsigned long end);
+
static int mlock_fixup(struct vm_area_struct * vma,
unsigned long start, unsigned long end, unsigned int newflags)
{
@@ -138,7 +142,7 @@
pages = (end - start) >> PAGE_SHIFT;
if (newflags & VM_LOCKED) {
pages = -pages;
- make_pages_present(start, end);
+ mlock_make_pages_present(vma, start, end);
}
vma->vm_mm->locked_vm -= pages;
}
--- linux-2.4.4-stock/mm/memory.c Tue May 8 17:25:36 2001
+++ linux/mm/memory.c Tue May 8 17:24:40 2001
@@ -1438,3 +1438,80 @@
} while (addr < end);
return 0;
}
+
+/*
+ * Specialized version of make_pages_present which does not require
+ * a multi-processor TLB purge for every page if nothing about the PTE
+ * was modified.
+ */
+int mlock_make_pages_present(struct vm_area_struct *vma,
+ unsigned long addr, unsigned long end)
+{
+ int ret, write;
+ struct mm_struct *mm = current->mm;
+
+ write = (vma->vm_flags & VM_WRITE) != 0;
+
+ /*
+ * We need the page table lock to synchronize with kswapd
+ * and the SMP-safe atomic PTE updates.
+ */
+ spin_lock(&mm->page_table_lock);
+
+ ret = 0;
+ for (ret=0; !ret && addr < end; addr += PAGE_SIZE) {
+ pgd_t *pgd;
+ pmd_t *pmd;
+ pte_t *pte, entry;
+ int modified;
+
+ current->state = TASK_RUNNING;
+ pgd = pgd_offset(mm, addr);
+ pmd = pmd_alloc(mm, pgd, addr);
+ if (!pmd) {
+ ret = -1;
+ break;
+ }
+ pte = pte_alloc(mm, pmd, addr);
+ if (!pte) {
+ ret = -1;
+ break;
+ }
+ entry = *pte;
+ if (!pte_present(entry)) {
+ /*
+ * If it truly wasn't present, we know that kswapd
+ * and the PTE updates will not touch it later. So
+ * drop the lock.
+ */
+ if (pte_none(entry)) {
+ ret = do_no_page(mm, vma, addr, write, pte);
+ continue;
+ }
+ ret = do_swap_page(mm, vma, addr, pte,
+ pte_to_swp_entry(entry), write);
+ continue;
+ }
+
+ modified = 0;
+ if (write) {
+ if (!pte_write(entry)) {
+ ret = do_wp_page(mm, vma, addr, pte, entry);
+ continue;
+ }
+ if (!pte_dirty(entry)) {
+ entry = pte_mkdirty(entry);
+ modified = 1;
+ }
+ }
+ if (!pte_young(entry)) {
+ entry = pte_mkyoung(entry);
+ modified = 1;
+ }
+ if (modified)
+ establish_pte(vma, addr, pte, entry);
+ }
+
+ spin_unlock(&mm->page_table_lock);
+ return ret;
+}
-
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