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

[Minchan Kim <minchan@kernel.org>]

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.

> 
> What should happen when the code encounters a VMA where
> MADV_FREE does not work?  Should it return an error?
> Should it skip over it?

I'd like to maintain it same as DONTNEED sematic so it could return
just EINVAL.

Thanks for the review!

-- 
Kind regards,
Minchan Kim

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