On Thu, 2008-12-18 at 06:10 -0500, Oren Laadan wrote:
> >> + mutex_lock(&mm->context.lock);
> >> +
> >> + hh->ldt_entry_size = LDT_ENTRY_SIZE;
> >> + hh->nldt = mm->context.size;
> >> +
> >> + cr_debug("nldt %d\n", hh->nldt);
> >> +
> >> + ret = cr_write_obj(ctx, &h, hh);
> >> + cr_hbuf_put(ctx, sizeof(*hh));
> >> + if (ret < 0)
> >> + goto out;
> >> +
> >> + ret = cr_kwrite(ctx, mm->context.ldt,
> >> + mm->context.size * LDT_ENTRY_SIZE);
> >
> > Do we really want to emit anything under lock? I realize that this
> > patch goes and does a ton of writes with mmap_sem held for read -- is
> > this ok?
>
> Because all tasks in the container must be frozen during the checkpoint,
> there is no performance penalty for keeping the locks. Although the object
> should not change in the interim anyways, the locks protects us from, e.g.
> the task unfreezing somehow, or being killed by the OOM killer, or any
> other change incurred from the "outside world" (even future code).
>
> Put in other words - in the long run it is safer to assume that the
> underlying object may otherwise change.
>
> (If we want to drop the lock here before cr_kwrite(), we need to copy the
> data to a temporary buffer first. If we also want to drop mmap_sem(), we
> need to be more careful with following the vma's.)
>
> Do you see a reason to not keeping the locks ?
Mike, although we're doing writes of the checkpoint file here, the *mm*
access is read-only. We only need really mmap_sem for write if we're
creating new VMAs, which we only do on restore. Was there an action
taken on the mm that would require a write that we missed?
Oren, I never considered the locking overhead, either. The fact that
the processes are frozen is very, very important here. The code is fine
as it stands because this *is* a very simple way to do it. But, this
probably deserves a comment.
-- Dave
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