linux-kernel - Re: [PATCH] mm: workingset: fix NULL ptr dereference

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Date:   Mon, 9 Apr 2018 18:12:11 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Chao Yu <yuchao0@...wei.com>, Jaegeuk Kim <jaegeuk@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        Chris Fries <cfries@...gle.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: workingset: fix NULL ptr dereference

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.

> > 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.