Re: [PATCH v3 02/13] pagewalk: improve vma handling

Naoya Horiguchi
2014-06-30T14:28:54Z

On Mon, Jun 30, 2014 at 02:53:11PM +0300, Kirill A. Shutemov wrote:
> On Fri, Jun 20, 2014 at 04:11:28PM -0400, Naoya Horiguchi wrote:
> > Current implementation of page table walker has a fundamental problem
> > in vma handling, which started when we tried to handle vma(VM_HUGETLB).
> > Because it's done in pgd loop, considering vma boundary makes code
> > complicated and bug-prone.
> >
> > From the users viewpoint, some user checks some vma-related condition to
> > determine whether the user really does page walk over the vma.
> >
> > In order to solve these, this patch moves vma check outside pgd loop and
> > introduce a new callback ->test_walk().
> >
> > ChangeLog v3:
> > - drop walk->skip control
> >
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > ---
> > include/linux/mm.h | 15 +++-
> > mm/pagewalk.c | 198 ++++++++++++++++++++++++++++++-----------------------
> > 2 files changed, 126 insertions(+), 87 deletions(-)
> >
> > diff --git v3.16-rc1.orig/include/linux/mm.h v3.16-rc1/include/linux/mm.h
> > index c5cb6394e6cb..489a63a06a4a 100644
> > --- v3.16-rc1.orig/include/linux/mm.h
> > +++ v3.16-rc1/include/linux/mm.h
> > @@ -1107,10 +1107,16 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> > * @pte_entry: if set, called for each non-empty PTE (4th-level) entry
> > * @pte_hole: if set, called for each hole at all levels
> > * @hugetlb_entry: if set, called for each hugetlb entry
> > - * *Caution*: The caller must hold mmap_sem() if @hugetlb_entry
> > - * is used.
> > + * @test_walk: caller specific callback function to determine whether
> > + * we walk over the current vma or not. A positive returned
> > + * value means "do page table walk over the current vma,"
> > + * and a negative one means "abort current page table walk
> > + * right now." 0 means "skip the current vma."
> > + * @mm: mm_struct representing the target process of page table walk
> > + * @vma: vma currently walked (NULL if walking outside vmas)
> > + * @private: private data for callbacks' usage
> > *
> > - * (see walk_page_range for more details)
> > + * (see the comment on walk_page_range() for more details)
> > */
> > struct mm_walk {
> > int (*pmd_entry)(pmd_t *pmd, unsigned long addr,
> > @@ -1122,7 +1128,10 @@ struct mm_walk {
> > int (*hugetlb_entry)(pte_t *pte, unsigned long hmask,
> > unsigned long addr, unsigned long next,
> > struct mm_walk *walk);
> > + int (*test_walk)(unsigned long addr, unsigned long next,
> > + struct mm_walk *walk);
> > struct mm_struct *mm;
> > + struct vm_area_struct *vma;
> > void *private;
> > };
> >
> > diff --git v3.16-rc1.orig/mm/pagewalk.c v3.16-rc1/mm/pagewalk.c
> > index 335690650b12..86d811202374 100644
> > --- v3.16-rc1.orig/mm/pagewalk.c
> > +++ v3.16-rc1/mm/pagewalk.c
> > @@ -59,7 +59,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> > continue;
> >
> > split_huge_page_pmd_mm(walk->mm, addr, pmd);
> > - if (pmd_none_or_trans_huge_or_clear_bad(pmd))
> > + if (pmd_trans_unstable(pmd))
> > goto again;
> > err = walk_pte_range(pmd, addr, next, walk);
> > if (err)
> > @@ -95,6 +95,32 @@ static int walk_pud_range(pgd_t *pgd, unsigned long addr, unsigned long end,
> > return err;
> > }
> >
> > +static int walk_pgd_range(unsigned long addr, unsigned long end,
> > + struct mm_walk *walk)
> > +{
> > + pgd_t *pgd;
> > + unsigned long next;
> > + int err = 0;
> > +
> > + pgd = pgd_offset(walk->mm, addr);
> > + do {
> > + next = pgd_addr_end(addr, end);
> > + if (pgd_none_or_clear_bad(pgd)) {
> > + if (walk->pte_hole)
> > + err = walk->pte_hole(addr, next, walk);
> > + if (err)
> > + break;
> > + continue;
> > + }
> > + if (walk->pmd_entry || walk->pte_entry)
> > + err = walk_pud_range(pgd, addr, next, walk);
> > + if (err)
> > + break;
> > + } while (pgd++, addr = next, addr != end);
> > +
> > + return err;
> > +}
> > +
> > #ifdef CONFIG_HUGETLB_PAGE
> > static unsigned long hugetlb_entry_end(struct hstate *h, unsigned long addr,
> > unsigned long end)
> > @@ -103,10 +129,10 @@ static unsigned long hugetlb_entry_end(struct hstate *h, unsigned long addr,
> > return boundary < end ? boundary : end;
> > }
> >
> > -static int walk_hugetlb_range(struct vm_area_struct *vma,
> > - unsigned long addr, unsigned long end,
> > +static int walk_hugetlb_range(unsigned long addr, unsigned long end,
> > struct mm_walk *walk)
> > {
> > + struct vm_area_struct *vma = walk->vma;
> > struct hstate *h = hstate_vma(vma);
> > unsigned long next;
> > unsigned long hmask = huge_page_mask(h);
> > @@ -119,15 +145,14 @@ static int walk_hugetlb_range(struct vm_area_struct *vma,
> > if (pte && walk->hugetlb_entry)
> > err = walk->hugetlb_entry(pte, hmask, addr, next, walk);
> > if (err)
> > - return err;
> > + break;
> > } while (addr = next, addr != end);
> >
> > return 0;
>
> I guess it should be 'return err;', right?
>
> > }
> >
> > #else /* CONFIG_HUGETLB_PAGE */
> > -static int walk_hugetlb_range(struct vm_area_struct *vma,
> > - unsigned long addr, unsigned long end,
> > +static int walk_hugetlb_range(unsigned long addr, unsigned long end,
> > struct mm_walk *walk)
> > {
> > return 0;
> > @@ -135,109 +160,114 @@ static int walk_hugetlb_range(struct vm_area_struct *vma,
> >
> > #endif /* CONFIG_HUGETLB_PAGE */
> >
> > +/*
> > + * Decide whether we really walk over the current vma on [@start, @end)
> > + * or skip it via the returned value. Return 0 if we do walk over the
> > + * current vma, and return 1 if we skip the vma. Negative values means
> > + * error, where we abort the current walk.
> > + *
> > + * Default check (only VM_PFNMAP check for now) is used when the caller
> > + * doesn't define test_walk() callback.
> > + */
> > +static int walk_page_test(unsigned long start, unsigned long end,
> > + struct mm_walk *walk)
> > +{
> > + struct vm_area_struct *vma = walk->vma;
> > +
> > + if (walk->test_walk)
> > + return walk->test_walk(start, end, walk);
> >
> > + /*
> > + * Do not walk over vma(VM_PFNMAP), because we have no valid struct
> > + * page backing a VM_PFNMAP range. See also commit a9ff785e4437.
> > + */
> > + if (vma->vm_flags & VM_PFNMAP)
> > + return 1;
> > + return 0;
> > +}
> > +
> > +static int __walk_page_range(unsigned long start, unsigned long end,
> > + struct mm_walk *walk)
> > +{
> > + int err = 0;
> > + struct vm_area_struct *vma = walk->vma;
> > +
> > + if (vma && is_vm_hugetlb_page(vma)) {
> > + if (walk->hugetlb_entry)
> > + err = walk_hugetlb_range(start, end, walk);
> > + } else
> > + err = walk_pgd_range(start, end, walk);
> > +
> > + return err;
> > +}
> >
> > /**
> > - * walk_page_range - walk a memory map's page tables with a callback
> > - * @addr: starting address
> > - * @end: ending address
> > - * @walk: set of callbacks to invoke for each level of the tree
> > - *
> > - * Recursively walk the page table for the memory area in a VMA,
> > - * calling supplied callbacks. Callbacks are called in-order (first
> > - * PGD, first PUD, first PMD, first PTE, second PTE... second PMD,
> > - * etc.). If lower-level callbacks are omitted, walking depth is reduced.
> > + * walk_page_range - walk page table with caller specific callbacks
> > *
> > - * Each callback receives an entry pointer and the start and end of the
> > - * associated range, and a copy of the original mm_walk for access to
> > - * the ->private or ->mm fields.
> > + * Recursively walk the page table tree of the process represented by @walk->mm
> > + * within the virtual address range [@start, @end). During walking, we can do
> > + * some caller-specific works for each entry, by setting up pmd_entry(),
> > + * pte_entry(), and/or hugetlb_entry(). If you don't set up for some of these
> > + * callbacks, the associated entries/pages are just ignored.
> > + * The return values of these callbacks are commonly defined like below:
> > + * - 0 : succeeded to handle the current entry, and if you don't reach the
> > + * end address yet, continue to walk.
> > + * - >0 : succeeded to handle the current entry, and return to the caller
> > + * with caller specific value.
> > + * - <0 : failed to handle the current entry, and return to the caller
> > + * with error code.
> > *
> > - * Usually no locks are taken, but splitting transparent huge page may
> > - * take page table lock. And the bottom level iterator will map PTE
> > - * directories from highmem if necessary.
> > + * Before starting to walk page table, some callers want to check whether
> > + * they really want to walk over the current vma, typically by checking
> > + * its vm_flags. walk_page_test() and @walk->test_walk() are used for this
> > + * purpose.
> > *
> > - * If any callback returns a non-zero value, the walk is aborted and
> > - * the return value is propagated back to the caller. Otherwise 0 is returned.
> > + * struct mm_walk keeps current values of some common data like vma and pmd,
> > + * which are useful for the access from callbacks. If you want to pass some
> > + * caller-specific data to callbacks, @walk->private should be helpful.
> > *
> > - * walk->mm->mmap_sem must be held for at least read if walk->hugetlb_entry
> > - * is !NULL.
> > + * Locking:
> > + * Callers of walk_page_range() and walk_page_vma() should hold
> > + * @walk->mm->mmap_sem, because these function traverse vma list and/or
> > + * access to vma's data.
> > */
> > -int walk_page_range(unsigned long addr, unsigned long end,
> > +int walk_page_range(unsigned long start, unsigned long end,
> > struct mm_walk *walk)
> > {
> > - pgd_t *pgd;
> > - unsigned long next;
> > int err = 0;
> > + unsigned long next;
> >
> > - if (addr >= end)
> > - return err;
> > + if (start >= end)
> > + return -EINVAL;
> >
> > if (!walk->mm)
> > return -EINVAL;
> >
> > VM_BUG_ON(!rwsem_is_locked(&walk->mm->mmap_sem));
> >
> > - pgd = pgd_offset(walk->mm, addr);
> > do {
> > - struct vm_area_struct *vma = NULL;
> > + struct vm_area_struct *vma;
> >
> > - next = pgd_addr_end(addr, end);
> > + vma = find_vma(walk->mm, start);
> > + if (!vma) { /* after the last vma */
> > + walk->vma = NULL;
> > + next = end;
> > + } else if (start < vma->vm_start) { /* outside the found vma */
> > + walk->vma = NULL;
> > + next = vma->vm_start;
>
> Is there a reason why we shoul go for __walk_page_range() for these two
> cases if walkj->pte_hole() is not defined?

Oh, I see, we can omit it.
I'll do it.

>
> Otherwise, looks okay.
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Thank you for your reviewing.

Naoya