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SubjectRe: fork: out of memory
On Tue, 25 Nov 1997, Rogier Wolff wrote:

> Alan Cox wrote:
> >
> > > These extra allocations would go to get_unused_fd() in fs/open.c.
> > > I didn't find any other place (but I just started working on it, it will
> > > take some time).
> >
> > The socket stuff allocates fd's. Im not sure it goes via fs/open.c properly
> > (yes it should)
> >
> > > Some preliminary searching showed me that I will have to modify at
> > > least 50 files!!! No big things, but absolutely neccessary if you ask
> > > me (unless I have problems in logic :)).
> >
> > That is a good sign that its time someone took the functions that
> > reference it directly and replaced them with one or two inline functions
> >
> > > When you say "atomically" do you mean "in one place" or with
> >
> > I mean "in a way that no other task on any processor can have accessed it
> > during the change"
>
> What's wrong with
>
>
> /* This piece of code allows concurrent read access to the fd
> array at all times. There are three lines of code that need
> to be protected against concurrent write access. These are
> protected by the "aquire/release_write lock". A mechanism
> for this needs to be in place anyway: we don't want another
> another thread allocating this very SAME fd concurrently.
> Don't we? */
>
> if (newfd >= files->cur_allocated) {
> if (newfd > files->cur_allocated) {
> /* Oops. Someone else must already be updating
> this, Not now baby.... I have a headache */
> return -ENOMEM;
> }
> newfds = kmalloc (1024 *sizeof (struct file *), GFP_KERNEL);
> if (!newfds) {
> return -ENOMEM;
> }
> aquire_write_lock (files);
> memcpy (newfds, files->fd,
> files->cur_allocated * sizeof (struct file *));
> /* This is the wrong moment to be changing the old
> structure. */
> files->fd = newfds;
> files->cur_allocated = 1024;
> release_write_lock (files);
> /* The old one could be freed, but it is connected to
> the files structure. */
> }
>
> Left as an excercise for the reader: change 1024 to "cur_allocated * 4"

Do you have to keep the old fd list around to handle code which may have a
pointer to it?

john alvord


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