[PATCH] D39464: Define fs::allocate_file which preallocates disk blocks.

[Kamil Rytarowski via llvm-commits]

On 14.11.2017 00:18, Rafael Avila de Espindola wrote:
> Kamil Rytarowski <n54 at gmx.com> writes:
> 
>> On 13.11.2017 19:21, Rafael Avila de Espindola wrote:
>>> Kamil Rytarowski <n54 at gmx.com> writes:
>>>
>>>> On 13.11.2017 18:19, Rafael Avila de Espindola wrote:
>>>>> Kamil Rytarowski via Phabricator <reviews at reviews.llvm.org> writes:
>>>>>
>>>>>> krytarowski added inline comments.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: llvm/lib/Support/Unix/Path.inc:436
>>>>>>        return std::error_code(Err, std::generic_category());
>>>>>> +    return make_error_code(errc::function_not_supported);
>>>>>>    }
>>>>>> ----------------
>>>>>> NetBSD needs `ftruncate`(2) as a fallback for `posix_fallocate`(2).
>>>>>>
>>>>>> In the default setup `posix_fallocate`() returns EOPNOTSUPP.
>>>>>
>>>>> Will ftruncate allocate space on netbsd? Note that it is OK for
>>>>> allocate_file to fail. The user then has to use a buffer and write(2)
>>>>> instead of mmap for output.
>>>>>
>>>>> Cheers,
>>>>> Rafael
>>>>>
>>>>
>>>> ftruncate(2)/NetBSD as an extension extends area and makes it
>>>> zero-filled. It's equivalent to lseek(2) + write(2).
>>>
>>> Cool, so an mmap of the file after a ftruncate will never crash because
>>> of disk full, correct?
>>>
>>> If so we could use use ftruncate on netbsd as a special case, similar to
>>> the special case we have for OS X.
>>>
>>> Cheers,
>>> Rafael
>>>
>>
>> I'm not that sure. Could you please provide a reproducer of a problem to
>> double check?
>>
>> I was trying to create a program that takes the whole space with
>> ftruncate(2) and mmap(2) over it works. I cannot take more space than
>> available, as I'm getting ENOSPC during ftruncate(2).
> 
> Try the following program on a fs with less than 1gb free:
> 
> ------------------------------------
>   #include <assert.h>
>   #include <fcntl.h>
>   #include <string.h>
>   #include <sys/mman.h>
>   #include <sys/stat.h>
>   #include <sys/types.h>
>   #include <unistd.h>
> 
>   int main(void) {
>     int fd = open("foobar", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
>     assert(fd != -1);
>     size_t size = 1 << 30;
>     int r = ftruncate(fd, size);
>     assert(r == 0);
>     void *addr = mmap(NULL, size, PROT_WRITE, MAP_SHARED, fd, 0);
>     assert(addr != MAP_FAILED);
>     memset(addr, 42, size);
>     r = munmap(addr, size);
>     assert(r == 0);
>     r = close(fd);
>     assert(r == 0);
>     return 0;
>   }
> ------------------------------------
> 
> Cheers,
> Rafael
> 

SIGSEGV (EFAULT) when attempting to execute the memset(3) line.

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