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

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Message-ID: <20180409183827.GD17558@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Mon, 9 Apr 2018 11:38:27 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.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>, 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 04/09, Minchan Kim wrote:
> On Mon, Apr 09, 2018 at 04:14:03AM -0700, Matthew Wilcox wrote:
> > On Mon, Apr 09, 2018 at 12:09:30PM +0900, Minchan Kim wrote:
> > > On Sun, Apr 08, 2018 at 07:49:25PM -0700, Matthew Wilcox wrote:
> > > > On Mon, Apr 09, 2018 at 10:58:15AM +0900, Minchan Kim wrote:
> > > > > It assumes shadow entry of radix tree relies on the init state
> > > > > that node->private_list allocated should be list_empty state.
> > > > > Currently, it's initailized in SLAB constructor which means
> > > > > node of radix tree would be initialized only when *slub allocates
> > > > > new page*, not *new object*. So, if some FS or subsystem pass
> > > > > gfp_mask to __GFP_ZERO, slub allocator will do memset blindly.
> > > > 
> > > > Wait, what?  Who's declaring their radix tree with GFP_ZERO flags?
> > > > I don't see anyone using INIT_RADIX_TREE or RADIX_TREE or RADIX_TREE_INIT
> > > > with GFP_ZERO.
> > > 
> > > Look at fs/f2fs/inode.c
> > > mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);
> > > 
> > > __add_to_page_cache_locked
> > >   radix_tree_maybe_preload
> > > 
> > > add_to_page_cache_lru
> > > 
> > > What's the wrong with setting __GFP_ZERO with mapping->gfp_mask?
> > 
> > Because it's a stupid thing to do.  Pages are allocated and then filled
> > from disk.  Zeroing them before DMAing to them is just a waste of time.
> 
> Every FSes do address_space to read pages from storage? I'm not sure.
> 
> If you're right, we need to insert WARN_ON to catch up __GFP_ZERO
> on mapping_set_gfp_mask at the beginning and remove all of those
> stupid thins. 
> 
> Jaegeuk, why do you need __GFP_ZERO? Could you explain?

Comment says "__GFP_ZERO returns a zeroed page on success."

The f2fs maintains two inodes to manage some metadata in the page cache,
which requires zeroed data when introducing a new structure. It's not
a big deal to avoid __GFP_ZERO for whatever performance reasons tho, does
it only matters with f2fs?

Thanks,