linux-kernel - Re: [PATCH v3 03/15] block, dax: fix lifetime of in-kernel dax mappings with dax_map

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Message-ID: <x494mh23nnz.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 03 Nov 2015 14:09:36 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	Dan Williams <dan.j.williams@...el.com>, axboe@...com,
	Jens Axboe <axboe@...nel.dk>, jack@...e.cz,
	linux-nvdimm@...1.01.org, david@...morbit.com,
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>, hch@....de
Subject: Re: [PATCH v3 03/15] block, dax: fix lifetime of in-kernel dax mappings with dax_map_atomic()

Ross Zwisler <ross.zwisler@...ux.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@...e.com>
>> Cc: Jens Axboe <axboe@...nel.dk>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Dave Chinner <david@...morbit.com>
>> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
>> Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.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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