Jan Kara wrote:
> On Thu 12-08-10 15:29:49, Joel Becker wrote:
>> diff --git a/fs/libfs.c b/fs/libfs.c
>> index dcaf972..f099566 100644
>> --- a/fs/libfs.c
>> +++ b/fs/libfs.c
>> @@ -955,6 +955,35 @@ int generic_file_fsync(struct file *file, int datasync)
>> }
>> EXPORT_SYMBOL(generic_file_fsync);
>>
>> +/**
>> + * generic_check_addressable - Check addressability of file system
>> + * @blocksize_bits: log of file system block size
>> + * @num_blocks: number of blocks in file system
>> + *
>> + * Determine whether a file system with @num_blocks blocks (and a
>> + * block size of 2**@blocksize_bits) is addressable by the sector_t
>> + * and page cache of the system. Return 0 if so and -EFBIG otherwise.
>> + */
>> +int generic_check_addressable(unsigned blocksize_bits, u64 num_blocks)
>> +{
>> + u64 last_fs_block = num_blocks - 1;
>> +
>> + if (unlikely(num_blocks == 0))
>> + return 0;
>> +
>> + if ((blocksize_bits < 9) || (blocksize_bits > PAGE_CACHE_SHIFT))
>> + return -EINVAL;
>> +
>> + if ((last_fs_block >
>> + (sector_t)(~0ULL) >> (blocksize_bits - 9)) ||
>> + (last_fs_block >
>> + (pgoff_t)(~0ULL) >> (PAGE_CACHE_SHIFT - blocksize_bits))) {
> ^^^ I don't get the pgoff_t check. Shouldn't it rather be
> (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)?
Argh that was my fault... Thankfully not too many 1k-blocksize-formatted
16T devices out there, I guess.
I went through the math again and also came up with:
total fs pages is blocks / (blocks per page)
total pages is blocks / (1 << PAGE_CACHE_SHIFT / 1 << blocksize_bits)
total pages is blocks / (1 << (PAGE_CACHE_SHIFT - blocksize_bits))
total pages is blocks * (1 >> (PAGE_CACHE_SHIFT - blocksize_bits))
total pages is blocks >> (PAGE_CACHE_SHIFT - blocksize_bits)
too big if total pages is > (pgoff_t)(~0ULL)
too big if blocks >> (PAGE_CACHE_SHIFT - blocksize_bits) > (pgoff_t)(~0ULL)
too big if blocks > (pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)
and to not overflow:
too big if blocks > (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)
so seems like the test is:
last_fs_block > (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)
Given the density of the logic in the helper it seems like maybe breaking it
up and adding specific comments might be helpful to the reader:
/* can IO layers fit total fs sectors in a sector_t? */
if (last_fs_block >
(sector_t)(~0ULL) >> (blocksize_bits - 9))
return -EFBIG;
/* can page cache fit total fs pages in a pgoff_t? */
if (last_fs_block >
(u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)
return -EFBIG;
Or something like that.
Sorry for chiming in late...
-Eric
> Because on 32-bit arch we are able to address 16TB device, which is for 1KB
> blocksize 1<<34 blocks. But your math gives 1<<30 blocks...
>
> Honza
>
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