Re: [PATCH] mm: Limit pgd range freeing to mm->task_size

[Catalin Marinas]

Hugh,

On Thu, Feb 14, 2013 at 09:24:09PM +0000, Hugh Dickins wrote:
> On Wed, 13 Feb 2013, Andrew Morton wrote:
> > On Wed, 13 Feb 2013 11:39:29 +0000
> > Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > 
> > > ARM processors with LPAE enabled use 3 levels of page tables, with an
> > > entry in the top level (pgd) covering 1GB of virtual space. Because of
> > > the branch relocation limitations on ARM, the loadable modules are
> > > mapped 16MB below PAGE_OFFSET, making the corresponding 1GB pgd shared
> > > between kernel modules and user space.
> > > 
> > > Since free_pgtables() is called with ceiling == 0, free_pgd_range() (and
> > > subsequently called functions) also frees the page table
> > > shared between user space and kernel modules (which is normally handled
> > > by the ARM-specific pgd_free() function).
> > > 
> > > This patch changes the ceiling argument to mm->task_size for the
> > > free_pgtables() and free_pgd_range() function calls. We cannot use
> > > TASK_SIZE since this macro may not be a run-time constant on 64-bit
> > > systems supporting compat applications.
> > 
> > I'm trying to work out why we're using 0 in there at all, rather than
> > ->task_size.  But that's lost in the mists of time.
> > 
> > As you've discovered, handling of task_size and TASK_SIZE is somewhat
> > inconsistent across architectures and with compat tasks.  I guess we
> > toss it in there and see if anything breaks...
> 
> ... and an x86_64 kernel quickly shows,
> with either 64-bit or 32-bit userspace, that exit_mmap() breaks at
> WARN_ON(mm->nr_ptes > (FIRST_USER_ADDRESS+PMD_SIZE-1)>>PMD_SHIFT);
> 
> We couldn't think of using mm->task_size in 2.6.12 because it didn't
> exist then; but although it sounds plausible, and on many architectures
> (x86_32?) it should be fine, in general it's not quite the right thing
> to use.  0 is an easy rounded-up-whatever-the-increment version of
> TASK_SIZE (okay, it's missing an implicit 1 before all its 0s).
> 
> The ceiling passed to free_pgtables() says how far up it can go in
> freeing pts and pmds and puds and pgds: when doing munmap(), you have
> to be careful not to stray beyond the range you're freeing; when doing
> exit_mmap(), you have to be careful to free all the areas you might
> have had to avoid before.

Yes, on ARM+LPAE we make sure we free what's left of the shared pgd (a
pmd page).

> mm->task_size does not necessarily fall on a nice boundary: use it
> instead of 0 and exit_mmap() is liable to leave unfreed page tables
> at several levels.
> 
> I'm sure that Catalin is right that he needs to adjust that ceiling arg
> to free_pgtables() to cope with a level shared between user and kernel.
> 
> I met the same problem two years ago, when doing a patch (which worked
> but went nowhere: x86 people kept on changing the early pagetable setup)
> to make CONFIG_VMSPLIT_2G_OPT and 3G_OPT compatible with CONFIG_X86_PAE.
> That shared a level beween user and kernel too: everything could be
> handled down in the arch code, except this free_pgtables() ceiling arg.
> 
> (I did not make any change to the free_pgd_range() calls in fs/exec.c,
> I'm not familiar with those at all: my patch appeared to work fine
> without touching them, but now I wonder.)
> 
> Here's the mm/mmap.c part of my patch (but it now looks like the
> default should go into include/asm-generic):

Thanks for the patch. It is related to FIRST_USER_ADDRESS which is
defined in asm/pgtable.h, so asm-generic/pgtable.h looks like a good
place. We can actually make FIRST_USER_ADDRESS generic as well since
apart from arm and unicore32 all the other architectures define it as 0.

I'll shortly post a series of two patches with your patch and the ARM
definition of USER_PGTABLES_CEILING.

-- 
Catalin
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