[PATCH] fadvise(2) system call

[John Baldwin]

On Saturday, October 29, 2011 5:40:58 pm Jilles Tjoelker wrote:
> On Fri, Oct 28, 2011 at 02:25:59PM -0400, John Baldwin wrote:
> > I have been working for the last week or so on a patch to add an
> > fadvise(2) system call.  It is somewhat similar to madvise(2) except
> > that it operates on a file descriptor instead of a memory region.  It
> > also only really makes sense for regular files and does not apply to
> > other file descriptor types.
> 
> Cool.
> 
> Various other posix_* functions such as posix_spawn() and posix_openpt()
> are implemented directly, not as a wrapper around s/posix_//. I think
> posix_madvise() is only implemented as a wrapper because madvise()
> already existed. Therefore, I don't see a reason why a function named
> fadvise would be useful on its own (except if there are existing
> applications that use that name).

Existing applications use the name and I find it ugly.  (I also wish we
had a plain fallocate() instead of just posix_fallocate().)  However, if
other folks prefer not having the wrapper I could update it to use the
posix_* name.

> If the advice is FADV_SEQUENTIAL, FADV_RANDOM or FADV_NOREUSE and the
> file descriptor is invalid (fget() fails), the struct fadvise_info is
> leaked.

Gah, fixed (I had thought of that case but forgot to update it when
converting the advice to be malloc'd vs stored in struct file directly).

> The comparisons
> 
> +		    (fa->fa_start != 0 && fa->fa_start == end + 1) ||
> +		    (uap->offset != 0 && fa->fa_end + 1 == uap->offset))) {
> 
> should instead be something like
> 
> +		    (end != OFF_MAX && fa->fa_start == end + 1) ||
> +		    (fa->fa_end != OFF_MAX && fa->fa_end + 1 == uap->offset))) {
> 
> to avoid integer overflow.

Hmm, but the expressions will still work in that case, yes?  I already
check for uap->offset and uap->len being negative earlier (so fa_start
and fa_end are always positive), and off_t is signed, so if end is
OFF_MAX, then end + 1 will certainly not == fa_start?

-- 
John Baldwin