On Wed 10-05-17 11:27:00, Ross Zwisler wrote:
> This is based on a patch from Jan Kara that fixed the equivalent race in
> the DAX PTE fault path.
>
> Currently DAX PMD read fault can race with write(2) in the following way:
>
> CPU1 - write(2) CPU2 - read fault
> dax_iomap_pmd_fault()
> ->iomap_begin() - sees hole
>
> dax_iomap_rw()
> iomap_apply()
> ->iomap_begin - allocates blocks
> dax_iomap_actor()
> invalidate_inode_pages2_range()
> - there's nothing to invalidate
>
> grab_mapping_entry()
> - we add huge zero page to the radix tree
> and map it to page tables
>
> The result is that hole page is mapped into page tables (and thus zeros
> are seen in mmap) while file has data written in that place.
>
> Fix the problem by locking exception entry before mapping blocks for the
> fault. That way we are sure invalidate_inode_pages2_range() call for
> racing write will either block on entry lock waiting for the fault to
> finish (and unmap stale page tables after that) or read fault will see
> already allocated blocks by write(2).
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Fixes: 9f141d6ef6258a3a37a045842d9ba7e68f368956
> CC: stable@xxxxxxxxxxxxxxx
> ---
>
> Jan, I just realized that we need an equivalent fix in the PMD path. Let's
> keep this with the rest of your series so they get applied together,
> applied to stable together, etc.
>
> This applies cleanly to the current linux/master (56868a460b83) + the four
> patches from Jan's series. I've run it through xfstests and some targeted
> testing for the PMD path.
Ah, right. Thanks for fixing it up. The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> fs/dax.c | 28 ++++++++++++++--------------
> 1 file changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 32f020c..93ae872 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1388,6 +1388,16 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> goto fallback;
>
> /*
> + * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX
> + * PMD or a HZP entry. If it can't (because a 4k page is already in
> + * the tree, for instance), it will return -EEXIST and we just fall
> + * back to 4k entries.
> + */
> + entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD);
> + if (IS_ERR(entry))
> + goto fallback;
> +
> + /*
> * Note that we don't use iomap_apply here. We aren't doing I/O, only
> * setting up a mapping, so really we're using iomap_begin() as a way
> * to look up our filesystem block.
> @@ -1395,21 +1405,11 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> pos = (loff_t)pgoff << PAGE_SHIFT;
> error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap);
> if (error)
> - goto fallback;
> + goto unlock_entry;
>
> if (iomap.offset + iomap.length < pos + PMD_SIZE)
> goto finish_iomap;
>
> - /*
> - * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX
> - * PMD or a HZP entry. If it can't (because a 4k page is already in
> - * the tree, for instance), it will return -EEXIST and we just fall
> - * back to 4k entries.
> - */
> - entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD);
> - if (IS_ERR(entry))
> - goto finish_iomap;
> -
> switch (iomap.type) {
> case IOMAP_MAPPED:
> result = dax_pmd_insert_mapping(vmf, &iomap, pos, &entry);
> @@ -1417,7 +1417,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> case IOMAP_UNWRITTEN:
> case IOMAP_HOLE:
> if (WARN_ON_ONCE(write))
> - goto unlock_entry;
> + break;
> result = dax_pmd_load_hole(vmf, &iomap, &entry);
> break;
> default:
> @@ -1425,8 +1425,6 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> break;
> }
>
> - unlock_entry:
> - put_locked_mapping_entry(mapping, pgoff, entry);
> finish_iomap:
> if (ops->iomap_end) {
> int copied = PMD_SIZE;
> @@ -1442,6 +1440,8 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> ops->iomap_end(inode, pos, PMD_SIZE, copied, iomap_flags,
> &iomap);
> }
> + unlock_entry:
> + put_locked_mapping_entry(mapping, pgoff, entry);
> fallback:
> if (result == VM_FAULT_FALLBACK) {
> split_huge_pmd(vma, vmf->pmd, vmf->address);
> --
> 2.9.3
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
