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    SubjectRe: [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX backed filesystem
    On Tue, Jan 29, 2019 at 8:54 AM <jglisse@redhat.com> wrote:
    >
    > From: Jérôme Glisse <jglisse@redhat.com>
    >
    > This add support to mirror vma which is an mmap of a file which is on
    > a filesystem that using a DAX block device. There is no reason not to
    > support that case.
    >

    The reason not to support it would be if it gets in the way of future
    DAX development. How does this interact with MAP_SYNC? I'm also
    concerned if this complicates DAX reflink support. In general I'd
    rather prioritize fixing the places where DAX is broken today before
    adding more cross-subsystem entanglements. The unit tests for
    filesystems (xfstests) are readily accessible. How would I go about
    regression testing DAX + HMM interactions?

    > Note that unlike GUP code we do not take page reference hence when we
    > back-off we have nothing to undo.
    >
    > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Dan Williams <dan.j.williams@intel.com>
    > Cc: Ralph Campbell <rcampbell@nvidia.com>
    > Cc: John Hubbard <jhubbard@nvidia.com>
    > ---
    > mm/hmm.c | 133 ++++++++++++++++++++++++++++++++++++++++++++++---------
    > 1 file changed, 112 insertions(+), 21 deletions(-)
    >
    > diff --git a/mm/hmm.c b/mm/hmm.c
    > index 8b87e1813313..1a444885404e 100644
    > --- a/mm/hmm.c
    > +++ b/mm/hmm.c
    > @@ -334,6 +334,7 @@ EXPORT_SYMBOL(hmm_mirror_unregister);
    >
    > struct hmm_vma_walk {
    > struct hmm_range *range;
    > + struct dev_pagemap *pgmap;
    > unsigned long last;
    > bool fault;
    > bool block;
    > @@ -508,6 +509,15 @@ static inline uint64_t pmd_to_hmm_pfn_flags(struct hmm_range *range, pmd_t pmd)
    > range->flags[HMM_PFN_VALID];
    > }
    >
    > +static inline uint64_t pud_to_hmm_pfn_flags(struct hmm_range *range, pud_t pud)
    > +{
    > + if (!pud_present(pud))
    > + return 0;
    > + return pud_write(pud) ? range->flags[HMM_PFN_VALID] |
    > + range->flags[HMM_PFN_WRITE] :
    > + range->flags[HMM_PFN_VALID];
    > +}
    > +
    > static int hmm_vma_handle_pmd(struct mm_walk *walk,
    > unsigned long addr,
    > unsigned long end,
    > @@ -529,8 +539,19 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk,
    > return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);
    >
    > pfn = pmd_pfn(pmd) + pte_index(addr);
    > - for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++)
    > + for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
    > + if (pmd_devmap(pmd)) {
    > + hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
    > + hmm_vma_walk->pgmap);
    > + if (unlikely(!hmm_vma_walk->pgmap))
    > + return -EBUSY;
    > + }
    > pfns[i] = hmm_pfn_from_pfn(range, pfn) | cpu_flags;
    > + }
    > + if (hmm_vma_walk->pgmap) {
    > + put_dev_pagemap(hmm_vma_walk->pgmap);
    > + hmm_vma_walk->pgmap = NULL;
    > + }
    > hmm_vma_walk->last = end;
    > return 0;
    > }
    > @@ -617,10 +638,24 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
    > if (fault || write_fault)
    > goto fault;
    >
    > + if (pte_devmap(pte)) {
    > + hmm_vma_walk->pgmap = get_dev_pagemap(pte_pfn(pte),
    > + hmm_vma_walk->pgmap);
    > + if (unlikely(!hmm_vma_walk->pgmap))
    > + return -EBUSY;
    > + } else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
    > + *pfn = range->values[HMM_PFN_SPECIAL];
    > + return -EFAULT;
    > + }
    > +
    > *pfn = hmm_pfn_from_pfn(range, pte_pfn(pte)) | cpu_flags;
    > return 0;
    >
    > fault:
    > + if (hmm_vma_walk->pgmap) {
    > + put_dev_pagemap(hmm_vma_walk->pgmap);
    > + hmm_vma_walk->pgmap = NULL;
    > + }
    > pte_unmap(ptep);
    > /* Fault any virtual address we were asked to fault */
    > return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);
    > @@ -708,12 +743,84 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
    > return r;
    > }
    > }
    > + if (hmm_vma_walk->pgmap) {
    > + put_dev_pagemap(hmm_vma_walk->pgmap);
    > + hmm_vma_walk->pgmap = NULL;
    > + }
    > pte_unmap(ptep - 1);
    >
    > hmm_vma_walk->last = addr;
    > return 0;
    > }
    >
    > +static int hmm_vma_walk_pud(pud_t *pudp,
    > + unsigned long start,
    > + unsigned long end,
    > + struct mm_walk *walk)
    > +{
    > + struct hmm_vma_walk *hmm_vma_walk = walk->private;
    > + struct hmm_range *range = hmm_vma_walk->range;
    > + struct vm_area_struct *vma = walk->vma;
    > + unsigned long addr = start, next;
    > + pmd_t *pmdp;
    > + pud_t pud;
    > + int ret;
    > +
    > +again:
    > + pud = READ_ONCE(*pudp);
    > + if (pud_none(pud))
    > + return hmm_vma_walk_hole(start, end, walk);
    > +
    > + if (pud_huge(pud) && pud_devmap(pud)) {
    > + unsigned long i, npages, pfn;
    > + uint64_t *pfns, cpu_flags;
    > + bool fault, write_fault;
    > +
    > + if (!pud_present(pud))
    > + return hmm_vma_walk_hole(start, end, walk);
    > +
    > + i = (addr - range->start) >> PAGE_SHIFT;
    > + npages = (end - addr) >> PAGE_SHIFT;
    > + pfns = &range->pfns[i];
    > +
    > + cpu_flags = pud_to_hmm_pfn_flags(range, pud);
    > + hmm_range_need_fault(hmm_vma_walk, pfns, npages,
    > + cpu_flags, &fault, &write_fault);
    > + if (fault || write_fault)
    > + return hmm_vma_walk_hole_(addr, end, fault,
    > + write_fault, walk);
    > +
    > + pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
    > + for (i = 0; i < npages; ++i, ++pfn) {
    > + hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
    > + hmm_vma_walk->pgmap);
    > + if (unlikely(!hmm_vma_walk->pgmap))
    > + return -EBUSY;
    > + pfns[i] = hmm_pfn_from_pfn(range, pfn) | cpu_flags;
    > + }
    > + if (hmm_vma_walk->pgmap) {
    > + put_dev_pagemap(hmm_vma_walk->pgmap);
    > + hmm_vma_walk->pgmap = NULL;
    > + }
    > + hmm_vma_walk->last = end;
    > + return 0;
    > + }
    > +
    > + split_huge_pud(vma, pudp, addr);
    > + if (pud_none(*pudp))
    > + goto again;
    > +
    > + pmdp = pmd_offset(pudp, addr);
    > + do {
    > + next = pmd_addr_end(addr, end);
    > + ret = hmm_vma_walk_pmd(pmdp, addr, next, walk);
    > + if (ret)
    > + return ret;
    > + } while (pmdp++, addr = next, addr != end);
    > +
    > + return 0;
    > +}
    > +
    > static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
    > unsigned long start, unsigned long end,
    > struct mm_walk *walk)
    > @@ -786,14 +893,6 @@ static void hmm_pfns_clear(struct hmm_range *range,
    > *pfns = range->values[HMM_PFN_NONE];
    > }
    >
    > -static void hmm_pfns_special(struct hmm_range *range)
    > -{
    > - unsigned long addr = range->start, i = 0;
    > -
    > - for (; addr < range->end; addr += PAGE_SIZE, i++)
    > - range->pfns[i] = range->values[HMM_PFN_SPECIAL];
    > -}
    > -
    > /*
    > * hmm_range_register() - start tracking change to CPU page table over a range
    > * @range: range
    > @@ -911,12 +1010,6 @@ long hmm_range_snapshot(struct hmm_range *range)
    > if (vma == NULL || (vma->vm_flags & device_vma))
    > return -EFAULT;
    >
    > - /* FIXME support dax */
    > - if (vma_is_dax(vma)) {
    > - hmm_pfns_special(range);
    > - return -EINVAL;
    > - }
    > -
    > if (is_vm_hugetlb_page(vma)) {
    > struct hstate *h = hstate_vma(vma);
    >
    > @@ -940,6 +1033,7 @@ long hmm_range_snapshot(struct hmm_range *range)
    > }
    >
    > range->vma = vma;
    > + hmm_vma_walk.pgmap = NULL;
    > hmm_vma_walk.last = start;
    > hmm_vma_walk.fault = false;
    > hmm_vma_walk.range = range;
    > @@ -951,6 +1045,7 @@ long hmm_range_snapshot(struct hmm_range *range)
    > mm_walk.pte_entry = NULL;
    > mm_walk.test_walk = NULL;
    > mm_walk.hugetlb_entry = NULL;
    > + mm_walk.pud_entry = hmm_vma_walk_pud;
    > mm_walk.pmd_entry = hmm_vma_walk_pmd;
    > mm_walk.pte_hole = hmm_vma_walk_hole;
    > mm_walk.hugetlb_entry = hmm_vma_walk_hugetlb_entry;
    > @@ -1018,12 +1113,6 @@ long hmm_range_fault(struct hmm_range *range, bool block)
    > if (vma == NULL || (vma->vm_flags & device_vma))
    > return -EFAULT;
    >
    > - /* FIXME support dax */
    > - if (vma_is_dax(vma)) {
    > - hmm_pfns_special(range);
    > - return -EINVAL;
    > - }
    > -
    > if (is_vm_hugetlb_page(vma)) {
    > struct hstate *h = hstate_vma(vma);
    >
    > @@ -1047,6 +1136,7 @@ long hmm_range_fault(struct hmm_range *range, bool block)
    > }
    >
    > range->vma = vma;
    > + hmm_vma_walk.pgmap = NULL;
    > hmm_vma_walk.last = start;
    > hmm_vma_walk.fault = true;
    > hmm_vma_walk.block = block;
    > @@ -1059,6 +1149,7 @@ long hmm_range_fault(struct hmm_range *range, bool block)
    > mm_walk.pte_entry = NULL;
    > mm_walk.test_walk = NULL;
    > mm_walk.hugetlb_entry = NULL;
    > + mm_walk.pud_entry = hmm_vma_walk_pud;
    > mm_walk.pmd_entry = hmm_vma_walk_pmd;
    > mm_walk.pte_hole = hmm_vma_walk_hole;
    > mm_walk.hugetlb_entry = hmm_vma_walk_hugetlb_entry;
    > --
    > 2.17.2
    >

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