[PATCH] O_NOATIME support for open(2)

[Konstantin Belousov]

On Sat, Aug 26, 2017 at 06:18:27PM +0200, Daniel Roethlisberger wrote:
> I'm trying to implement O_NOATIME support for open(2) in order to
> provide a more elegant way for backup/archiving software to
> prevent atime clobbering.  Except for a 2008 thread on this list
> I did not find any material; not sure if anybody is interested in
> this or if there are reasons why this was never implemented.
Please point out the thread, e.g. by providing a link to the first
message in the thread in mailman archive.

> 
> The attached patch against 11.1 implements O_NOATIME support for
> open(2); it prevents read(2) and mmap(2) from clobbering atime if
> the file descriptor was opened with O_NOATIME.  O_NOATIME is only
> permitted for root and the owner of the file.  Currently it is
> only implemented for ufs/ffs.  It seems to work for me but has
> not been extensively tested.
What would happen when additional page-in occurs on the mmaped area ?

> 
> I am interested in feedback from people who know their way around
> I/O and VFS code before I extend this to other file systems, make
> O_NOATIME tunable by fcntl(2), wire it to the Linux compat layer
> and write docs.  Does the implementation look sane?  Did I miss
> something important?
> 
> Specifically, is there a better way to pass O_NOATIME into
> vm_mmap_vnode other than adding an additional boolean_t argument?
> I did not use an additional mmap flag because that would have
> required additional logic to prevent userland from passing the
> flag to the mmap syscall.
If you need two booleans to the function, consider substituting the
arguments with the single u_int flags, and define two flags, one for
the writecounted, one for noatime.