Re: [PATCH] mm: workingset: fix NULL ptr dereference

[Minchan Kim]

On Mon, Apr 09, 2018 at 06:12:11PM -0700, Matthew Wilcox wrote:
> On Tue, Apr 10, 2018 at 08:04:09AM +0900, Minchan Kim wrote:
> > On Mon, Apr 09, 2018 at 08:20:32AM -0700, Matthew Wilcox wrote:
> > > I don't think this is something the radix tree should know about.
> > 
> > Because shadow entry implementation is hidden by radix tree implemetation.
> > IOW, radix tree user cannot know how it works.
> 
> I have no idea what you mean.
> 
> > > SLAB should be checking for it (the patch I posted earlier in this
> > 
> > I don't think it's right approach. SLAB constructor can initialize
> > some metadata for slab page populated as well as page zeroing.
> > However, __GFP_ZERO means only clearing pages, not metadata.
> > So it's different semantic. No need to mix out.
> 
> No, __GFP_ZERO is specified to clear the allocated memory whether
> you're allocating from alloc_pages or from slab.  What makes no sense
> is allocating an object from slab with a constructor *and* __GFP_ZERO.
> They're in conflict, and slab can't fulfill both of those requirements.

It's a stable material. If you really think it does make sense,
please submit patch separately.

> 
> > > thread), but the right place to filter this out is in the caller of
> > > radix_tree_maybe_preload -- it's already filtering out HIGHMEM pages,
> > > and should filter out GFP_ZERO too.
> > 
> > radix_tree_[maybe]_preload is exported API, which are error-prone
> > for out of modules or upcoming customers.
> > 
> > More proper place is __radix_tree_preload.
> 
> I could not disagree with you more.  It is the responsibility of the
> callers of radix_tree_preload to avoid calling it with nonsense flags
> like __GFP_DMA, __GFP_HIGHMEM or __GFP_ZERO.

How about this?

It would fix current problem and warn potential bugs as well.
radix_tree_preload already has done such warning and
radix_tree_maybe_preload has skipping for misbehaivor gfp.