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SubjectRe: [PATCH v3 03/15] block, dax: fix lifetime of in-kernel dax mappings with dax_map_atomic()
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Ross Zwisler <ross.zwisler@linux.intel.com> writes:

> On Sun, Nov 01, 2015 at 11:29:58PM -0500, Dan Williams wrote:
>> The DAX implementation needs to protect new calls to ->direct_access()
>> and usage of its return value against unbind of the underlying block
>> device. Use blk_queue_enter()/blk_queue_exit() to either prevent
>> blk_cleanup_queue() from proceeding, or fail the dax_map_atomic() if the
>> request_queue is being torn down.
>>
>> Cc: Jan Kara <jack@suse.com>
>> Cc: Jens Axboe <axboe@kernel.dk>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Dave Chinner <david@fromorbit.com>
>> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>> Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> ---
> <>
>> @@ -42,9 +76,9 @@ int dax_clear_blocks(struct inode *inode, sector_t block, long size)
>> long count, sz;
>>
>> sz = min_t(long, size, SZ_1M);
>> - count = bdev_direct_access(bdev, sector, &addr, &pfn, sz);
>> - if (count < 0)
>> - return count;
>> + addr = __dax_map_atomic(bdev, sector, size, &pfn, &count);
>
> I think you can use dax_map_atomic() here instead, allowing you to avoid
> having a local pfn variable that otherwise goes unused.

But __dax_map_atomic doesn't return the count, and I believe that is
what's used.

>> @@ -138,21 +176,27 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
>> bh->b_size -= done;
>> }
>>
>> - hole = iov_iter_rw(iter) != WRITE && !buffer_written(bh);
>> + hole = rw == READ && !buffer_written(bh);
>> if (hole) {
>> addr = NULL;
>> size = bh->b_size - first;
>> } else {
>> - retval = dax_get_addr(bh, &addr, blkbits);
>> - if (retval < 0)
>> + dax_unmap_atomic(bdev, kmap);
>> + kmap = __dax_map_atomic(bdev,
>> + to_sector(bh, inode),
>> + bh->b_size, &pfn, &map_len);
>
> Same as above, you can use dax_map_atomic() here instead and nix the pfn variable.

same as above. ;-)

>> @@ -305,11 +353,10 @@ static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
>> goto out;
>> }
>>
>> - error = bdev_direct_access(bh->b_bdev, sector, &addr, &pfn, bh->b_size);
>> - if (error < 0)
>> - goto out;
>> - if (error < PAGE_SIZE) {
>> - error = -EIO;
>> + addr = __dax_map_atomic(bdev, to_sector(bh, inode), bh->b_size,
>> + &pfn, NULL);
>> + if (IS_ERR(addr)) {
>> + error = PTR_ERR(addr);
>
> Just a note that we lost the check for bdev_direct_access() returning less
> than PAGE_SIZE. Are we sure this can't happen and that it's safe to remove
> the check?

Yes, it's safe, I checked during my review. This page size assumption
is present throughout the file, and makes reviewing it very frustrating
for the uninitiated. I think that's worth a follow-on cleanup patch.

Cheers,
Jeff

>> @@ -609,15 +655,20 @@ 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) {
>> + long length;
>> + unsigned long pfn;
>> + void __pmem *kaddr = __dax_map_atomic(bdev,
>> + to_sector(&bh, inode), HPAGE_SIZE, &pfn,
>> + &length);
>
> Let's use PMD_SIZE instead of HPAGE_SIZE to be consistent with the rest of the
> DAX code.
>
>> +
>> + if (IS_ERR(kaddr)) {
>> result = VM_FAULT_SIGBUS;
>> goto out;
>> }
>> - if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
>> + if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR)) {
>> + dax_unmap_atomic(bdev, kaddr);
>> goto fallback;
>> + }
>>
>> if (buffer_unwritten(&bh) || buffer_new(&bh)) {
>> clear_pmem(kaddr, HPAGE_SIZE);
>
> Ditto, let's use PMD_SIZE for consistency (I realize this was changed ealier
> in the series).


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