linux-kernel - Re: [PATCH] mm/slub: Add a dump_stack() to the unexpected GFP check

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Date:   Mon, 16 Jan 2017 10:59:25 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, Vlastimil Babka <vbabka@...e.cz>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/slub: Add a dump_stack() to the unexpected GFP check

On Mon 16-01-17 11:48:51, Leon Romanovsky wrote:
> On Mon, Jan 16, 2017 at 10:37:02AM +0100, Borislav Petkov wrote:
> > On Mon, Jan 16, 2017 at 11:28:40AM +0200, Leon Romanovsky wrote:
> > > On Mon, Jan 16, 2017 at 10:16:43AM +0100, Borislav Petkov wrote:
> > > > From: Borislav Petkov <bp@...e.de>
> > > >
> > > > We wanna know who's doing such a thing. Like slab.c does that.
> > > >
> > > > Signed-off-by: Borislav Petkov <bp@...e.de>
> > > > ---
> > > >  mm/slub.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > >
> > > > diff --git a/mm/slub.c b/mm/slub.c
> > > > index 067598a00849..1b0fa7625d6d 100644
> > > > --- a/mm/slub.c
> > > > +++ b/mm/slub.c
> > > > @@ -1623,6 +1623,7 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
> > > >  		flags &= ~GFP_SLAB_BUG_MASK;
> > > >  		pr_warn("Unexpected gfp: %#x (%pGg). Fixing up to gfp: %#x (%pGg). Fix your code!\n",
> > > >  				invalid_mask, &invalid_mask, flags, &flags);
> > > > +		dump_stack();
> > >
> > > Will it make sense to change these two lines above to WARN(true, .....)?
> >
> > Should be equivalent.
> 
> Almost, except one point - pr_warn and dump_stack have different log
> levels. There is a chance that user won't see pr_warn message above, but
> dump_stack will be always present.
> 
> For WARN_XXX, users will always see message and stack at the same time.

On the other hand WARN* will taint the kernel and this sounds a bit
overreacting for something like a wrong gfp mask which is perfectly
recoverable. Not to mention users who care configured to panic on
warning.

So while I do not have a strong opinion on this I would rather stay with
the dump_stack.


-- 
Michal Hocko
SUSE Labs