Re: [PATCH] mm/kfence: reset PG_slab and memcg_data before freeing __kfence_pool

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, May 05, 2022 at 09:19:31AM +0200, Marco Elver wrote:
> On Thu, 5 May 2022 at 09:12, Marco Elver <elver@google.com> wrote:
> >
> > On Thu, 5 May 2022 at 09:01, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> > >
> > > When kfence fails to initialize kfence pool, it frees the pool.
> > > But it does not reset PG_slab flag and memcg_data of struct page.
> > >
> > > Below is a BUG because of this. Let's fix it by resetting PG_slab
> > > and memcg_data before free.
> > >
> > > [    0.089149] BUG: Bad page state in process swapper/0  pfn:3d8e06
> > > [    0.089149] page:ffffea46cf638180 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x3d8e06
> > > [    0.089150] memcg:ffffffff94a475d1
> > > [    0.089150] flags: 0x17ffffc0000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
> > > [    0.089151] raw: 0017ffffc0000200 ffffea46cf638188 ffffea46cf638188 0000000000000000
> > > [    0.089152] raw: 0000000000000000 0000000000000000 00000000ffffffff ffffffff94a475d1
> > > [    0.089152] page dumped because: page still charged to cgroup
> > > [    0.089153] Modules linked in:
> > > [    0.089153] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G    B   W         5.18.0-rc1+ #965
> > > [    0.089154] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> > > [    0.089154] Call Trace:
> > > [    0.089155]  <TASK>
> > > [    0.089155]  dump_stack_lvl+0x49/0x5f
> > > [    0.089157]  dump_stack+0x10/0x12
> > > [    0.089158]  bad_page.cold+0x63/0x94
> > > [    0.089159]  check_free_page_bad+0x66/0x70
> > > [    0.089160]  __free_pages_ok+0x423/0x530
> > > [    0.089161]  __free_pages_core+0x8e/0xa0
> > > [    0.089162]  memblock_free_pages+0x10/0x12
> > > [    0.089164]  memblock_free_late+0x8f/0xb9
> > > [    0.089165]  kfence_init+0x68/0x92
> > > [    0.089166]  start_kernel+0x789/0x992
> > > [    0.089167]  x86_64_start_reservations+0x24/0x26
> > > [    0.089168]  x86_64_start_kernel+0xa9/0xaf
> > > [    0.089170]  secondary_startup_64_no_verify+0xd5/0xdb
> > > [    0.089171]  </TASK>
> >
> > This is probably:
> >
> > Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
> 
> Hmm, looking closer at the above BUG, I think it's
> 
> Fixes: 8f0b36497303 ("mm: kfence: fix objcgs vector allocation")
> 
> ?

Marco, Thanks for comments.

I think it fixes both because not clearing PG_slab also invokes a BUG.

> 
> > > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > > ---
> > >  mm/kfence/core.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > >
> > > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > > index a203747ad2c0..2ab3d473321e 100644
> > > --- a/mm/kfence/core.c
> > > +++ b/mm/kfence/core.c
> > > @@ -642,6 +642,13 @@ static bool __init kfence_init_pool_early(void)
> > >          * fails for the first page, and therefore expect addr==__kfence_pool in
> > >          * most failure cases.
> > >          */
> > > +       for (char *p = (char *)addr; p < __kfence_pool + KFENCE_POOL_SIZE; p += PAGE_SIZE) {
> > > +               struct page *page;
> > > +
> > > +               page = virt_to_page(p);
> >
> > #ifdef CONFIG_MEMCG
> >
> > > +               page->memcg_data = 0;
> >
> > #endif
> >
> > > +               __ClearPageSlab(page);
> >

Ah, thanks! Will do in v2.

> > We're now using __folio_set_slab(), so I'm guessing this should be
> > __folio_clear_slab()?

Right. Will do in v2.

> >
> > > +       }
> > >         memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
> > >         __kfence_pool = NULL;
> > >         return false;
> > > --
> > > 2.32.0
> > >

-- 
Thanks,
Hyeonggon