Re: [bug report] mm: avoid leaving partial pfn mappings around in error case

[Lorenzo Stoakes]

+ get_maintainers.pl people for drivers/misc/sgi-gru/grumain.c

On Sun, Sep 15, 2024 at 03:09:35PM GMT, Dan Carpenter wrote:
> On Sun, Sep 15, 2024 at 01:01:43PM +0100, Lorenzo Stoakes wrote:
> > On Sun, Sep 15, 2024 at 01:08:27PM GMT, Dan Carpenter wrote:
> > > Hi Linus,
> > >
> > > Commit 79a61cc3fc04 ("mm: avoid leaving partial pfn mappings around in
> > > error case") from Sep 11, 2024 (linux-next), leads to the following
> > > Smatch static checker warning:
> > >
> > > 	mm/memory.c:2709 remap_pfn_range_notrack()
> > > 	warn: sleeping in atomic context
> > >
> > > mm/memory.c
> > >     2696 int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
> > >     2697                 unsigned long pfn, unsigned long size, pgprot_t prot)
> > >     2698 {
> > >     2699         int error = remap_pfn_range_internal(vma, addr, pfn, size, prot);
> > >     2700
> > >     2701         if (!error)
> > >     2702                 return 0;
> > >     2703
> > >     2704         /*
> > >     2705          * A partial pfn range mapping is dangerous: it does not
> > >     2706          * maintain page reference counts, and callers may free
> > >     2707          * pages due to the error. So zap it early.
> > >     2708          */
> > > --> 2709         zap_page_range_single(vma, addr, size, NULL);
> > >
> > > The lru_add_drain() function at the start of zap_page_range_single() takes a
> > > mutext.
> >
> > Hm does it? I see a local lock, and some folio batch locking which are
> > local locks too?
>
> Ah...  No it doesn't.  It's the mmu_notifier_invalidate_range_start() which is
> a might_sleep() function.  Sorry for the confusion.

OK so in conclusion it seems to be that Linus's commit introducing
zap_page_range_single() accidentally had smatch hit a might_sleep() via
mmu_notifier_invalidate_range_start(), but it should, in theory, have fired
due to page table allocations invoking the page allocator that might sleep,
but didn't, because smatch misses the below might_alloc() path...

-> prepare_alloc_pages()
   -> might_alloc()
      -> might_sleep_if(gfpflags_allow_blocking(gfp_mask))

...as a result of get_zeroed_page() tripping it up *breathes*. :)

(please correct me if I am wrong here).

The preempt_disable() is introduced in commit fe5bb6b00c3a9 ("sgi-gru: misc
GRU cleanup") from... 2009, but it fixed it from the far far more broken
'disable preemption before taking a mutex' situation that existed before.

So fix seems to me to not invoke remap_pfn_range() with preemption disabled
and a mutex held? gru_fault() maintainers added for input...

>
> regards,
> dan carpenter
>
>