sys_fallocate support in glibc

[Avantika Mathur <mathur at linux dot vnet dot ibm dot com>]

The sys_fallocate() system call was added to the Linux kernel earlier 
this year.  I am planning on adding support for the system call to 
glibc.  Is this work is already in progress?  If it is in progress, I 
would be happy to help, otherwise I will start working on support for 
sys_fallocate.

There are also plans to change the functionality of posix_fallocate to 
first call sys_fallocate() to perform preallocation, and fall back to 
the library implementation if the syscall is not supported in the 
filesystem.  I will be working on these changes as well.

I have included the description of the system call below.

Thank you,
Avantika Mathur

sys_fallocate - preallocate blocks or free preallocated blocks
@fd: the file descriptor
@mode: mode specifies if fallocate should preallocate blocks OR free
    (unallocate) preallocated blocks. Currently only FA_ALLOCATE and
     FA_DEALLOCATE modes are supported.
@offset: The offset within file, from where (un)allocation is being
       requested. It should not have a negative value.
@len: The amount (in bytes) of space to be (un)allocated, from the offset.

This system call, depending on the mode, preallocates or unallocates blocks
for a file. The range of blocks depends on the value of offset and len
arguments provided by the user/application. For FA_ALLOCATE mode, if this
system call succeeds, subsequent writes to the file in the given range
(specified by offset & len) should not fail - even if the file system
later becomes full. Hence the preallocation done is persistent (valid
even after reopen of the file and remount/reboot).

Note: In case the file system does not support preallocation,
posix_fallocate() should fall back to the library implementation (i.e.
allocating zero-filled new blocks to the file).

Return Values
   0    : On SUCCESS a value of zero is returned.
   error    : On Failure, an error code will be returned.
An error code of -ENOSYS or -EOPNOTSUPP should make posix_fallocate()
fall back on library implementation of fallocate.