On Tue, Sep 22, 2015 at 10:46 AM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> The following commit:
>
> commit 46c043ede471 ("mm: take i_mmap_lock in unmap_mapping_range() for
> DAX")
>
> moved some code in __dax_pmd_fault() that was responsible for zeroing
> newly allocated PMD pages. The new location didn't properly set up
> 'kaddr', though, so when run this code resulted in a NULL pointer BUG.
>
> Fix this by getting the correct 'kaddr' via bdev_direct_access(), and
> only make the second call to bdev_direct_access() if we don't already
> have a PFN from the first call.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
> fs/dax.c | 31 ++++++++++++++++++++++---------
> 1 file changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 7ae6df7..08ac2bd 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -532,7 +532,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> void __pmem *kaddr;
> pgoff_t size, pgoff;
> sector_t block, sector;
> - unsigned long pfn;
> + unsigned long pfn = 0;
> int result = 0;
>
> /* Fall back to PTEs if we're going to COW */
> @@ -569,8 +569,20 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
> goto fallback;
>
> + sector = bh.b_blocknr << (blkbits - 9);
> +
> if (buffer_unwritten(&bh) || buffer_new(&bh)) {
> int i;
> +
> + length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn,
> + bh.b_size);
> + if (length < 0) {
> + result = VM_FAULT_SIGBUS;
> + goto out;
> + }
> + if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
> + goto fallback;
> +
> for (i = 0; i < PTRS_PER_PMD; i++)
> clear_pmem(kaddr + i * PAGE_SIZE, PAGE_SIZE);
> wmb_pmem();
> @@ -623,15 +635,16 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> result = VM_FAULT_NOPAGE;
> spin_unlock(ptl);
> } else {
> - sector = bh.b_blocknr << (blkbits - 9);
> - length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn,
> - bh.b_size);
> - if (length < 0) {
> - result = VM_FAULT_SIGBUS;
> - goto out;
> + if (pfn == 0) {
> + length = bdev_direct_access(bh.b_bdev, sector, &kaddr,
> + &pfn, bh.b_size);
bdev_direct_access isn't that expensive just call it twice
unconditionally. We'll be going that direction anyways when we fix
the lifetime of 'kaddr' race in DAX.
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