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

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Date:   Tue, 10 Apr 2018 08:39:46 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michal Hocko <mhocko@...nel.org>, Jaegeuk Kim <jaegeuk@...nel.org>,
        Minchan Kim <minchan@...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>, Jan Kara <jack@...e.cz>,
        Chris Fries <cfries@...gle.com>, Chao Yu <yuchao0@...wei.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 05:05:28AM -0700, Matthew Wilcox wrote:
> On Tue, Apr 10, 2018 at 10:26:43AM +0200, Michal Hocko wrote:
> > On Mon 09-04-18 12:40:44, Matthew Wilcox wrote:
> > > The problem is that the mapping gfp flags are used not only for allocating
> > > pages, but also for allocating the page cache data structures that hold
> > > the pages.  F2FS is the only filesystem that set the __GFP_ZERO bit,
> > > so it's the first time anyone's noticed that the page cache passes the
> > > __GFP_ZERO bit through to the radix tree allocation routines, which
> > > causes the radix tree nodes to be zeroed instead of constructed.
> > > 
> > > I think the right solution to this is:
> > 
> > This just hides the underlying problem that the node is not fully and
> > properly initialized. Relying on the previous released state is just too
> > subtle.
> 
> That's the fundamental design of slab-with-constructors.  The user provides
> a constructor, so all newly allocagted objects are initialised to a known
> state, then the user will restore the object to that state when it frees
> the object to slab.

Fully agreed. I'm surprised there is so much pushback to what Willy is
saying here, this stuff has been established for years.

> > Are you going to blacklist all potential gfp flags that come
> > from the mapping? This is just unmaintainable! If anything this should
> > be an explicit & with the allowed set of allowed flags.
> 
> Oh, I agree that using the set of flags used to allocate the page
> in order to allocate the radix tree nodes is a pretty horrible idea.
> 
> Your suggestion, then, is:
> 
> -	error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
> +	error = radix_tree_preload(gfp_mask & GFP_RECLAIM_MASK);

That looks reasonable to me.