Re: [PATCH 06/17] arm: mmu_gather rework

[Russell King]

On Mon, Feb 28, 2011 at 11:59:07AM +0000, Russell King wrote:
> It may be hacky but then the TLB shootdown interface is hacky too.  We
> don't keep the vma around to re-use after tlb_end_vma() - if you think
> that then you misunderstand what's going on.  The vma pointer is kept
> around as a cheap way of allowing tlb_finish_mmu() to distinguish
> between the unmap_region() mode and the shift_arg_pages() mode.

As I think I mentioned, the TLB shootdown interface either needs rewriting
from scratch as its currently a broken design, or it needs tlb_gather_mmu()
to take a proper mode argument, rather than this useless 'fullmm' argument
which only gives half the story.

The fact is that the interface has three modes, and distinguishing between
them requires a certain amount of black magic.  Explicitly, the !fullmm
case has two modes, and it requires implementations to remember whether
tlb_start_vma() has been called before tlb_finish_mm() or not.

Maybe this will help you understand the ARM implementation - this doesn't
change the functionality, but may make things clearer.

diff --git a/arch/arm/include/asm/tlb.h b/arch/arm/include/asm/tlb.h
index 82dfe5d..73fb813 100644
--- a/arch/arm/include/asm/tlb.h
+++ b/arch/arm/include/asm/tlb.h
@@ -54,7 +54,7 @@
 struct mmu_gather {
 	struct mm_struct	*mm;
 	unsigned int		fullmm;
-	struct vm_area_struct	*vma;
+	unsigned int		byvma;
 	unsigned long		range_start;
 	unsigned long		range_end;
 	unsigned int		nr;
@@ -68,23 +68,18 @@ DECLARE_PER_CPU(struct mmu_gather, mmu_gathers);
  * code is used:
  *  1. Unmapping a range of vmas.  See zap_page_range(), unmap_region().
  *     tlb->fullmm = 0, and tlb_start_vma/tlb_end_vma will be called.
- *     tlb->vma will be non-NULL.
+ *     tlb->byvma will be true.
  *  2. Unmapping all vmas.  See exit_mmap().
  *     tlb->fullmm = 1, and tlb_start_vma/tlb_end_vma will be called.
- *     tlb->vma will be non-NULL.  Additionally, page tables will be freed.
+ *     tlb->byvma will be true.  Additionally, page tables will be freed.
  *  3. Unmapping argument pages.  See shift_arg_pages().
  *     tlb->fullmm = 0, but tlb_start_vma/tlb_end_vma will not be called.
- *     tlb->vma will be NULL.
+ *     tlb->byvma will be false.
  */
 static inline void tlb_flush(struct mmu_gather *tlb)
 {
-	if (tlb->fullmm || !tlb->vma)
+	if (tlb->fullmm || !tlb->byvma)
 		flush_tlb_mm(tlb->mm);
-	else if (tlb->range_end > 0) {
-		flush_tlb_range(tlb->vma, tlb->range_start, tlb->range_end);
-		tlb->range_start = TASK_SIZE;
-		tlb->range_end = 0;
-	}
 }
 
 static inline void tlb_add_flush(struct mmu_gather *tlb, unsigned long addr)
@@ -113,7 +108,7 @@ tlb_gather_mmu(struct mm_struct *mm, unsigned int full_mm_flush)
 
 	tlb->mm = mm;
 	tlb->fullmm = full_mm_flush;
-	tlb->vma = NULL;
+	tlb->byvma = 0;
 	tlb->nr = 0;
 
 	return tlb;
@@ -149,7 +144,7 @@ tlb_start_vma(struct mmu_gather *tlb, struct vm_area_struct *vma)
 {
 	if (!tlb->fullmm) {
 		flush_cache_range(vma, vma->vm_start, vma->vm_end);
-		tlb->vma = vma;
+		tlb->byvma = 1;
 		tlb->range_start = TASK_SIZE;
 		tlb->range_end = 0;
 	}
@@ -158,8 +153,11 @@ tlb_start_vma(struct mmu_gather *tlb, struct vm_area_struct *vma)
 static inline void
 tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vma)
 {
-	if (!tlb->fullmm)
-		tlb_flush(tlb);
+	if (!tlb->fullmm && tlb->range_end > 0) {
+		flush_tlb_range(vma, tlb->range_start, tlb->range_end);
+		tlb->range_start = TASK_SIZE;
+		tlb->range_end = 0;
+	}
 }
 
 static inline void tlb_remove_page(struct mmu_gather *tlb, struct page *page)

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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