mremap: avoid sending one IPI per page (7b6efc2b) · Commits · 戴 / test

This replaces ptep_clear_flush() with ptep_get_and_clear() and a single flush_tlb_range() at the end of the loop, to avoid sending one IPI for each page. The mmu_notifier_invalidate_range_start/end section is enlarged accordingly but this is not going to fundamentally change things. It was more by accident that the region under mremap was for the most part still available for secondary MMUs: the primary MMU was never allowed to reliably access that region for the duration of the mremap (modulo trapping SIGSEGV on the old address range which sounds unpractical and flakey). If users wants secondary MMUs not to lose access to a large region under mremap they should reduce the mremap size accordingly in userland and run multiple calls. Overall this will run faster so it's actually going to reduce the time the region is under mremap for the primary MMU which should provide a net benefit to apps. For KVM this is a noop because the guest physical memory is never mremapped, there's just no point it ever moving it while guest runs. One target of this optimization is JVM GC (so unrelated to the mmu notifier logic). Signed-off-by:

Andrea Arcangeli <aarcange@redhat.com> Acked-by:

Johannes Weiner <jweiner@redhat.com> Acked-by:

Mel Gorman <mgorman@suse.de> Acked-by:

Rik van Riel <riel@redhat.com> Cc: Hugh Dickins <hughd@google.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

mm/mremap.c

+9 −6

Original line number	Diff line number	Diff line
		@@ -80,11 +80,7 @@ static void move_ptes(struct vm_area_struct vma, pmd_t old_pmd,
		struct mm_struct *mm = vma->vm_mm;
		pte_t old_pte, new_pte, pte;
		spinlock_t old_ptl, new_ptl;
		unsigned long old_start;

		old_start = old_addr;
		mmu_notifier_invalidate_range_start(vma->vm_mm,
		old_start, old_end);
		if (vma->vm_file) {
		/*
		* Subtle point from Rajesh Venkatasubramanian: before
		@@ -111,7 +107,7 @@ static void move_ptes(struct vm_area_struct vma, pmd_t old_pmd,
		new_pte++, new_addr += PAGE_SIZE) {
		if (pte_none(*old_pte))
		continue;
		pte = ptep_clear_flush(vma, old_addr, old_pte);
		pte = ptep_get_and_clear(mm, old_addr, old_pte);
		pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
		set_pte_at(mm, new_addr, new_pte, pte);
		}
		@@ -123,7 +119,6 @@ static void move_ptes(struct vm_area_struct vma, pmd_t old_pmd,
		pte_unmap_unlock(old_pte - 1, old_ptl);
		if (mapping)
		mutex_unlock(&mapping->i_mmap_mutex);
		mmu_notifier_invalidate_range_end(vma->vm_mm, old_start, old_end);
		}

		#define LATENCY_LIMIT (64 * PAGE_SIZE)
		@@ -134,10 +129,13 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
		{
		unsigned long extent, next, old_end;
		pmd_t old_pmd, new_pmd;
		bool need_flush = false;

		old_end = old_addr + len;
		flush_cache_range(vma, old_addr, old_end);

		mmu_notifier_invalidate_range_start(vma->vm_mm, old_addr, old_end);

		for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
		cond_resched();
		next = (old_addr + PMD_SIZE) & PMD_MASK;
		@@ -158,7 +156,12 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
		extent = LATENCY_LIMIT;
		move_ptes(vma, old_pmd, old_addr, old_addr + extent,
		new_vma, new_pmd, new_addr);
		need_flush = true;
		}
		if (likely(need_flush))
		flush_tlb_range(vma, old_end-len, old_addr);

		mmu_notifier_invalidate_range_end(vma->vm_mm, old_end-len, old_end);

		return len + old_addr - old_end; /* how much done */
		}

Admin message