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Date: Fri, 26 Sep 2014 16:33:26 -0400
From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To: Peter Feiner <pfeiner@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
"Kirill A. Shutemov" <kirill@...temov.name>,
Cyrill Gorcunov <gorcunov@...nvz.org>,
Pavel Emelyanov <xemul@...allels.com>,
Jamie Liu <jamieliu@...gle.com>,
Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm: softdirty: unmapped addresses between VMAs are
clean
On Mon, Sep 15, 2014 at 11:40:38AM -0700, Peter Feiner wrote:
> If a /proc/pid/pagemap read spans a [VMA, an unmapped region, then a
> VM_SOFTDIRTY VMA], the virtual pages in the unmapped region are reported
> as softdirty. Here's a program to demonstrate the bug:
>
> int main() {
> const uint64_t PAGEMAP_SOFTDIRTY = 1ul << 55;
> uint64_t pme[3];
> int fd = open("/proc/self/pagemap", O_RDONLY);;
> char *m = mmap(NULL, 3 * getpagesize(), PROT_READ,
> MAP_ANONYMOUS | MAP_SHARED, -1, 0);
> munmap(m + getpagesize(), getpagesize());
> pread(fd, pme, 24, (unsigned long) m / getpagesize() * 8);
> assert(pme[0] & PAGEMAP_SOFTDIRTY); /* passes */
> assert(!(pme[1] & PAGEMAP_SOFTDIRTY)); /* fails */
> assert(pme[2] & PAGEMAP_SOFTDIRTY); /* passes */
> return 0;
> }
>
> (Note that all pages in new VMAs are softdirty until cleared).
>
> Tested:
> Used the program given above. I'm going to include this code in
> a selftest in the future.
>
> Signed-off-by: Peter Feiner <pfeiner@...gle.com>
I triggered the BUG_ON(is_vm_hugetlb_page(vma)) introduced by this patch,
when I simply read /proc/pid/pagemap of the process using hugetlb.
This BUG_ON looks right itself, but find_vma() can find vmas beyond
the pmd boundary, so checking the overrun is necessary.
Could you test and merge the following change?
---
From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Date: Fri, 26 Sep 2014 15:57:39 -0400
Subject: [PATCH] pagemap: prevent pagemap_pte_range() from overrunning
When the vm_end address of the last vma just before vma(VM_HUGETLB)
is not aligned to PMD boundary, the while loop in pagemap_pte_range()
gets vma(VM_HUGETLB) and triggers BUG_ON(is_vm_hugetlb_page(vma)).
This patch fixes it by checking the overrun.
Fixes: 62c98294410d ("mm: softdirty: unmapped addresses between VMAs are clean")
Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
---
fs/proc/task_mmu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 5674675adeae..f2b15da32a7f 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1083,7 +1083,7 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
return err;
}
- if (!vma)
+ if (!vma || vma->vm_start >= end)
break;
/*
* We can't possibly be in a hugetlb VMA. In general,
--
1.9.3
Thanks,
Naoya Horiguchi
> ---
>
> v1 -> v2:
> Restructured patch to make logic more clear.
> ---
> fs/proc/task_mmu.c | 61 +++++++++++++++++++++++++++++++++++-------------------
> 1 file changed, 40 insertions(+), 21 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index dfc791c..2abf37b 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1020,7 +1020,6 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> spinlock_t *ptl;
> pte_t *pte;
> int err = 0;
> - pagemap_entry_t pme = make_pme(PM_NOT_PRESENT(pm->v2));
>
> /* find the first VMA at or above 'addr' */
> vma = find_vma(walk->mm, addr);
> @@ -1034,6 +1033,7 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>
> for (; addr != end; addr += PAGE_SIZE) {
> unsigned long offset;
> + pagemap_entry_t pme;
>
> offset = (addr & ~PAGEMAP_WALK_MASK) >>
> PAGE_SHIFT;
> @@ -1048,32 +1048,51 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>
> if (pmd_trans_unstable(pmd))
> return 0;
> - for (; addr != end; addr += PAGE_SIZE) {
> - int flags2;
> -
> - /* check to see if we've left 'vma' behind
> - * and need a new, higher one */
> - if (vma && (addr >= vma->vm_end)) {
> - vma = find_vma(walk->mm, addr);
> - if (vma && (vma->vm_flags & VM_SOFTDIRTY))
> - flags2 = __PM_SOFT_DIRTY;
> - else
> - flags2 = 0;
> - pme = make_pme(PM_NOT_PRESENT(pm->v2) | PM_STATUS2(pm->v2, flags2));
> +
> + while (1) {
> + /* End of address space hole, which we mark as non-present. */
> + unsigned long hole_end;
> +
> + if (vma)
> + hole_end = min(end, vma->vm_start);
> + else
> + hole_end = end;
> +
> + for (; addr < hole_end; addr += PAGE_SIZE) {
> + pagemap_entry_t pme = make_pme(PM_NOT_PRESENT(pm->v2));
> +
> + err = add_to_pagemap(addr, &pme, pm);
> + if (err)
> + return err;
> }
>
> - /* check that 'vma' actually covers this address,
> - * and that it isn't a huge page vma */
> - if (vma && (vma->vm_start <= addr) &&
> - !is_vm_hugetlb_page(vma)) {
> + if (!vma)
> + break;
> + /*
> + * We can't possibly be in a hugetlb VMA. In general,
> + * for a mm_walk with a pmd_entry and a hugetlb_entry,
> + * the pmd_entry can only be called on addresses in a
> + * hugetlb if the walk starts in a non-hugetlb VMA and
> + * spans a hugepage VMA. Since pagemap_read walks are
> + * PMD-sized and PMD-aligned, this will never be true.
> + */
> + BUG_ON(is_vm_hugetlb_page(vma));
> +
> + /* Addresses in the VMA. */
> + for (; addr < min(end, vma->vm_end); addr += PAGE_SIZE) {
> + pagemap_entry_t pme;
> pte = pte_offset_map(pmd, addr);
> pte_to_pagemap_entry(&pme, pm, vma, addr, *pte);
> - /* unmap before userspace copy */
> pte_unmap(pte);
> + err = add_to_pagemap(addr, &pme, pm);
> + if (err)
> + return err;
> }
> - err = add_to_pagemap(addr, &pme, pm);
> - if (err)
> - return err;
> +
> + if (addr == end)
> + break;
> +
> + vma = find_vma(walk->mm, addr);
> }
>
> cond_resched();
> --
> 2.1.0.rc2.206.gedb03e5
>
> --
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