Re: [PATCH] BUG() in exec_mmap()

[Ernie Petrides]

On Friday, 10-Oct-2003 at 7:57 -0300, Marcelo Tosatti wrote:

> On Thu, 9 Oct 2003, Ernie Petrides wrote:
>
> > On Thursday, 9-Oct-2003 at 17:47 -0300, Marcelo Tosatti wrote:
> >
> > > On Thu, 9 Oct 2003, Mika Penttilä wrote:
> > >
> > > > Hmm.. you still need to mmput(old_mm) etc, just remove the mm_users == 1
> > > > optimization from the beginning of exec_mmap, so this patch is wrong!
> > >
> > > Right. Ill fix it up by hand.
> >
> > Mika is correct that the exit_mmap(old_mm) still needs to happen on the
> > last use of the "mm_struct".  But whether it's called directly from
> > exec_mmap() or indirectly from mmput() still needs to depend on the
> > value of "mm_users".
> >
> > The original logic avoided the mmdrop(active_mm) call if there was an
> > old_mm, so I'd infer that the mm_struct reference count is not bumped
> > twice for both references from the task_struct (mm and active_mm).  So
> > the patch would need to be reworked to avoid the double decrement, too.
>
> I dont get you, sorry (I'm not a real expert on that piece of code,
> so...).
>
> From what I understand the "if (old_mm && mm_users == 1)" if case is just
> an optimization to avoid the allocation of a new mm structure.
>
> The functionality will be the same without that piece of code, it will
> just be slower.

I've checked your -pre7 patch now, and I see that the 2nd block of code
was added back in.  So what you've got now should work fine.


> > Is it possible to just use down_read(&old_mm->mmap_sem) and
> > up_read(&old_mm->mmap_sem) inside exec_mmap() around the optimized call
> > to exit_mmap() instead?
>
> Doing that locking inside exit_mmap() not feasible IMO... it might be too
> expensive.

Okay, sounds good.  Thanks for the follow-up.


Cheers.  -ernie
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