Chris Wilson wrote:
> When using remap_pfn_range() from a fault handler, we are exposed to
> races between concurrent faults. Rather than hitting a BUG, report the
> error back to the caller, like vm_insert_pfn().
>
> v2: Fix the pte address for unmapping along the error path.
> v3: Report the error back and cleanup partial remaps.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
>
> Whilst this has the semantics I want to allow two concurrent, but
> serialised, pagefaults that try to prefault the same object to succeed,
> it looks fragile and fraught with subtlety.
> -Chris
>
> ---
> mm/memory.c | 54 ++++++++++++++++++++++++++++++++++++++----------------
> 1 file changed, 38 insertions(+), 16 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index d67fd9f..be51fcc 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1657,32 +1657,41 @@ EXPORT_SYMBOL(vm_insert_mixed);
> * in null mappings (currently treated as "copy-on-access")
> */
> static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
> - unsigned long addr, unsigned long end,
> - unsigned long pfn, pgprot_t prot)
> + unsigned long addr, unsigned long end,
> + unsigned long pfn, pgprot_t prot,
> + bool first)
> {
With this long parameter list, wouldn't it cleaner to pass down a point to
structure instead? This could simplify the code, I believe.
> pte_t *pte;
> spinlock_t *ptl;
> + int err = 0;
>
> pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
> if (!pte)
> return -ENOMEM;
> arch_enter_lazy_mmu_mode();
> do {
> - BUG_ON(!pte_none(*pte));
> + if (!pte_none(*pte)) {
> + err = first ? -EBUSY : -EINVAL;
> + pte++;
> + break;
> + }
> + first = false;
> set_pte_at(mm, addr, pte, pte_mkspecial(pfn_pte(pfn, prot)));
> pfn++;
> } while (pte++, addr += PAGE_SIZE, addr != end);
> arch_leave_lazy_mmu_mode();
> pte_unmap_unlock(pte - 1, ptl);
> - return 0;
> + return err;
> }
>
> static inline int remap_pmd_range(struct mm_struct *mm, pud_t *pud,
> - unsigned long addr, unsigned long end,
> - unsigned long pfn, pgprot_t prot)
> + unsigned long addr, unsigned long end,
> + unsigned long pfn, pgprot_t prot,
> + bool first)
> {
> pmd_t *pmd;
> unsigned long next;
> + int err;
>
> pfn -= addr >> PAGE_SHIFT;
> pmd = pmd_alloc(mm, pud, addr);
> @@ -1691,19 +1700,23 @@ static inline int remap_pmd_range(struct mm_struct *mm, pud_t *pud,
> VM_BUG_ON(pmd_trans_huge(*pmd));
> do {
> next = pmd_addr_end(addr, end);
> - if (remap_pte_range(mm, pmd, addr, next,
> - pfn + (addr >> PAGE_SHIFT), prot))
> - return -ENOMEM;
> + err = remap_pte_range(mm, pmd, addr, next,
> + pfn + (addr >> PAGE_SHIFT), prot, first);
> + if (err)
> + return err;
> +
> + first = false;
> } while (pmd++, addr = next, addr != end);
> return 0;
> }
>
> static inline int remap_pud_range(struct mm_struct *mm, pgd_t *pgd,
> - unsigned long addr, unsigned long end,
> - unsigned long pfn, pgprot_t prot)
> + unsigned long addr, unsigned long end,
> + unsigned long pfn, pgprot_t prot, bool first)
> {
> pud_t *pud;
> unsigned long next;
> + int err;
>
> pfn -= addr >> PAGE_SHIFT;
> pud = pud_alloc(mm, pgd, addr);
> @@ -1711,9 +1724,12 @@ static inline int remap_pud_range(struct mm_struct *mm, pgd_t *pgd,
> return -ENOMEM;
> do {
> next = pud_addr_end(addr, end);
> - if (remap_pmd_range(mm, pud, addr, next,
> - pfn + (addr >> PAGE_SHIFT), prot))
> - return -ENOMEM;
> + err = remap_pmd_range(mm, pud, addr, next,
> + pfn + (addr >> PAGE_SHIFT), prot, first);
> + if (err)
> + return err;
> +
> + first = false;
> } while (pud++, addr = next, addr != end);
> return 0;
> }
> @@ -1735,6 +1751,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
> unsigned long next;
> unsigned long end = addr + PAGE_ALIGN(size);
> struct mm_struct *mm = vma->vm_mm;
> + bool first = true;
> int err;
>
> /*
> @@ -1774,13 +1791,18 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
> do {
> next = pgd_addr_end(addr, end);
> err = remap_pud_range(mm, pgd, addr, next,
> - pfn + (addr >> PAGE_SHIFT), prot);
> + pfn + (addr >> PAGE_SHIFT), prot, first);
> if (err)
> break;
> +
> + first = false;
> } while (pgd++, addr = next, addr != end);
>
> - if (err)
> + if (err) {
> untrack_pfn(vma, pfn, PAGE_ALIGN(size));
> + if (err != -EBUSY)
> + zap_page_range_single(vma, addr, size, NULL);
Hm. If I read it correctly, you zap whole range, not only what you've
set up. Looks wrong.
And for after zap, you probably whant to return -EBUSY to caller of
remap_pfn_range(), not -EINVAL.
> + }
>
> return err;
> }
> --
> 1.9.1
>
--
Kirill A. Shutemov
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