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

Eric Sandeen
2020-02-25T23:33:44Z

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