svn commit: r250027 - head/sys/kern

[Scott Long]

On May 6, 2013, at 2:05 PM, Konstantin Belousov <kostikbel at gmail.com> wrote:

> On Mon, May 06, 2013 at 08:16:11PM +0200, Pawel Jakub Dawidek wrote:
>> On Sun, Apr 28, 2013 at 07:12:09PM +0000, Konstantin Belousov wrote:
>>> Author: kib
>>> Date: Sun Apr 28 19:12:09 2013
>>> New Revision: 250027
>>> URL: http://svnweb.freebsd.org/changeset/base/250027
>>> 
>>> Log:
>>>  Eliminate the layering violation in the kern_sendfile().  When quering
>>>  the file size, use VOP_GETATTR() instead of accessing vnode vm_object
>>>  un_pager.vnp.vnp_size.
>> 
>> Doesn't it add extra I/O to query file system about file's attributes?
>> If it does, does it matter here?
> 
> It should not, typically.

Whenever you say the word "typically", I get nervous.  I take this to mean that under
pressure, the syscall may likely synchronously block to complete this call.  If so,
then it makes sendfile() pretty much unusable for us at Netflix, and it means that
we will either need to revert this or resort to a hack.

> 
> E.g. UFS always keeps the unpacked inode in memory for any non-reclaimed
> vnode.  For NFS, vnode usually caches the attributes.
> 
> Anyway, using VOP_GETATTR() is the only sanctioned way to get file size.
> Directly accessing vm_object assumes that vm_object is of OBJT_VNODE type,
> which is no longer true.

How about just testing for this assumption and only performing the VOP_GETATTR
if it's not true?

Scott