Re: [PATCH V4] libxfs: use FALLOC_FL_ZERO_RANGE in libxfs_device_zero

[Eric Sandeen <sandeen@sandeen.net>]

On 2/25/20 11:16 AM, Darrick J. Wong wrote:
> On Tue, Feb 25, 2020 at 10:13:55AM -0800, Eric Sandeen wrote:
>> I had a request from someone who cared about mkfs speed over
>> a slower network block device to look into using faster zeroing
>> methods, particularly for the log, during mkfs.
>>
>> Using FALLOC_FL_ZERO_RANGE is faster in this case than writing
>> a bunch of zeros across a wire.
>>
>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>> ---
>>
>> V2: Clean up all the nasty stuff I'd flung out there as a wild first
>> cut, thanks Dave.
>>
>> V3: make len_bytes a size_t; leave "end_offset" where it is for the loop
>> use.  It's a bit odd but ... just don't mess with it for now, one patch
>> one change.
>>
>> V4: Use EOPNOTSUPP not EOPNOTSUP (same on linux anyway but meh)
>> I ignored(tm) darrick's suggestion to make libxfs_device_zero accept
>> a longer length, for now - no callers need anything bigger at this time.
> 
> But... 
> 

...

>> diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
>> index 0d9d7202..e2d9d790 100644
>> --- a/libxfs/rdwr.c
>> +++ b/libxfs/rdwr.c
>> @@ -61,8 +61,18 @@ libxfs_device_zero(struct xfs_buftarg *btp, xfs_daddr_t start, uint len)
>>  {
>>  	xfs_off_t	start_offset, end_offset, offset;
>>  	ssize_t		zsize, bytes;
>> +	size_t		len_bytes;
>>  	char		*z;
>> -	int		fd;
>> +	int		error, fd;
>> +
>> +	fd = libxfs_device_to_fd(btp->dev);
>> +	start_offset = LIBXFS_BBTOOFF64(start);
>> +
>> +	/* try to use special zeroing methods, fall back to writes if needed */
>> +	len_bytes = LIBXFS_BBTOOFF64(len);
> 
> ...but if the caller passes in (say) 2^23 daddrs on a 32-bit system,
> this conversion will try to stuff 2^32 into a size_t (which is 32-bit),
> causing an integer overflow.  I grok that no callers currently try this,
> but this seems like leaving a logic bomb that could go off on what are
> becoming difficult-to-test architectures.

That's true, but ....

> Granted the added overflow checking and whatnot required to convert that
> last parameter of libxfs_device_zero to unsigned long long could very
> well justify a separate patch for fixing the 64-bitness of the whole
> api.
> 
> --D 
> 
>> +	error = platform_zero_range(fd, start_offset, len_bytes);
>> +	if (!error)
>> +		return 0;
>>  
>>  	zsize = min(BDSTRAT_SIZE, BBTOB(len));

it's an existing problem, right?  One which this patch doesn't address.  :)

-Eric