Re: [PATCH v5] mm: support madvise(MADV_FREE)

[Minchan Kim <minchan@kernel.org>]

On Fri, May 09, 2014 at 11:12:01AM -0400, Rik van Riel wrote:
> On 05/09/2014 02:28 AM, Minchan Kim wrote:
> >On Fri, May 09, 2014 at 03:17:14PM +0900, Minchan Kim wrote:
> >>Hello Rik,
> >>
> >>On Thu, May 08, 2014 at 04:04:33PM -0400, Rik van Riel wrote:
> >>>On 04/20/2014 09:56 PM, Minchan Kim wrote:
> >>>
> >>>>In summary, MADV_FREE is about 2 time faster than MADV_DONTNEED.
> >>>
> >>>This is awesome.
> >>
> >>Thanks!
> >>
> >>>
> >>>I have a few nitpicks with the patch, though :)
> >>>
> >>>>+static long madvise_lazyfree(struct vm_area_struct *vma,
> >>>>+			     struct vm_area_struct **prev,
> >>>>+			     unsigned long start, unsigned long end)
> >>>>+{
> >>>>+	*prev = vma;
> >>>>+	if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
> >>>>+		return -EINVAL;
> >>>>+
> >>>>+	/* MADV_FREE works for only anon vma at the moment */
> >>>>+	if (vma->vm_file)
> >>>>+		return -EINVAL;
> >>>>+
> >>>>+	lazyfree_range(vma, start, end - start);
> >>>>+	return 0;
> >>>>+}
> >>>
> >>>This code checks whether lazyfree_range would work on
> >>>the VMA...
> >>>
> >>>>diff --git a/mm/memory.c b/mm/memory.c
> >>>>index c4b5bc250820..ca427f258204 100644
> >>>>--- a/mm/memory.c
> >>>>+++ b/mm/memory.c
> >>>>@@ -1270,6 +1270,104 @@ static inline unsigned long zap_pud_range(struct mmu_gather *tlb,
> >>>>  	return addr;
> >>>>  }
> >>>>
> >>>>+static unsigned long lazyfree_pte_range(struct mmu_gather *tlb,
> >>>>+				struct vm_area_struct *vma, pmd_t *pmd,
> >>>>+				unsigned long addr, unsigned long end)
> >>>>+{
> >>>>+	struct mm_struct *mm = tlb->mm;
> >>>>+	spinlock_t *ptl;
> >>>>+	pte_t *start_pte;
> >>>>+	pte_t *pte;
> >>>>+
> >>>>+	start_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> >>>>+	pte = start_pte;
> >>>>+	arch_enter_lazy_mmu_mode();
> >>>>+	do {
> >>>>+		pte_t ptent = *pte;
> >>>>+
> >>>>+		if (pte_none(ptent))
> >>>>+			continue;
> >>>>+
> >>>>+		if (!pte_present(ptent))
> >>>>+			continue;
> >>>>+
> >>>>+		ptent = pte_mkold(ptent);
> >>>>+		ptent = pte_mkclean(ptent);
> >>>>+		set_pte_at(mm, addr, pte, ptent);
> >>>>+		tlb_remove_tlb_entry(tlb, pte, addr);
> >>>
> >>>This may not work on PPC, which has a weird hash table for
> >>>its TLB. You will find that tlb_remove_tlb_entry does
> >>>nothing for PPC64, and set_pte_at does not remove the hash
> >>>table entry either.
> >>
> >>Hmm, I didn't notice that. Thanks Rik.
> >>
> >>Maybe I need this in asm-generic.
> >>
> >>static inline void ptep_set_lazyfree(struct mm_struct *mm, unsigned addr, pte_t *ptep)
> >>{
> >>         pte_t ptent = *ptep;
> >>         ptent = pte_mkold(ptent);
> >>         ptent = pte_mkclean(ptent);
> >>         set_pte_at(mm, addr, ptep, ptent);
> >>}
> >>
> >>For arch/powerpc/include/asm/pgtable.h
> >>
> >>static inline void ptep_set_lazyfree(struct mm_struct *mm, unsigned long addr,
> >>                         pte_t *ptep)
> >>{
> >>         pte_update(mm, addr, ptep, _PAGE_DIRTY|_PAGE_ACCESSED, 0, 0);
> >>}
> >>
> >>>
> >>>>@@ -1370,6 +1485,31 @@ void unmap_vmas(struct mmu_gather *tlb,
> >>>>  }
> >>>>
> >>>>  /**
> >>>>+ * lazyfree_range - clear dirty bit of pte in a given range
> >>>>+ * @vma: vm_area_struct holding the applicable pages
> >>>>+ * @start: starting address of pages
> >>>>+ * @size: number of bytes to do lazyfree
> >>>>+ *
> >>>>+ * Caller must protect the VMA list
> >>>>+ */
> >>>>+void lazyfree_range(struct vm_area_struct *vma, unsigned long start,
> >>>>+		unsigned long size)
> >>>>+{
> >>>>+	struct mm_struct *mm = vma->vm_mm;
> >>>>+	struct mmu_gather tlb;
> >>>>+	unsigned long end = start + size;
> >>>>+
> >>>>+	lru_add_drain();
> >>>>+	tlb_gather_mmu(&tlb, mm, start, end);
> >>>>+	update_hiwater_rss(mm);
> >>>>+	mmu_notifier_invalidate_range_start(mm, start, end);
> >>>>+	for ( ; vma && vma->vm_start < end; vma = vma->vm_next)
> >>>>+		lazyfree_single_vma(&tlb, vma, start, end);
> >>>>+	mmu_notifier_invalidate_range_end(mm, start, end);
> >>>>+	tlb_finish_mmu(&tlb, start, end);
> >>>>+}
> >>>
> >>>This function, called by madvise_lazyfree, can iterate
> >>>over multiple VMAs.
> >>>
> >>>However, madvise_lazyfree only checked one of them.
> >>
> >>Oops, the check should have been lazyfree_range.
> >>Will fix.
> >
> >Now that I see the code, madvise_vma always pass *a* vma so madvise_lazyfree
> >doesn't cover multiple vma all at once so the current sematic is same with
> >dontneed. So, I don't see any problem. If I miss something, let me know it.
> >
> 
> Does that mean lazyfree_range is unnecessary, and everything
> can be done inside lazyfree_single_vma ?

Yeb, Will resend.
Thanks!

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>