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

[Matthew Wilcox]

On Mon, Apr 09, 2018 at 11:49:58PM +0900, Minchan Kim wrote:
> On Mon, Apr 09, 2018 at 08:25:06PM +0800, Chao Yu wrote:
> > On 2018/4/9 19:25, 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:
> > >>> 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
> > 
> > No, sometimes, we need to write meta data to new allocated block address,
> > then we will allocate a zeroed page in inner inode's address space, and
> > fill partial data in it, and leave other place with zero value which means
> > some fields are initial status.
> 
> Thanks for the explaining.
> 
> > There are two inner inodes (meta inode and node inode) setting __GFP_ZERO,
> > I have just checked them, for both of them, we can avoid using __GFP_ZERO,
> > and do initialization by ourselves to avoid unneeded/redundant zeroing
> > from mm.
> 
> Yub, it would be desirable for f2fs. Please go ahead for f2fs side.
> However, I think current problem is orthgonal. Now, the problem is
> radix_tree_node allocation is bind to page cache allocation.
> Why does FS cannot allocate page cache with __GFP_ZERO?
> I agree if the concern is only performance matter as Matthew mentioned.
> But it is beyond that because it shouldn't do due to limitation
> of workingset shadow entry implementation. I think such coupling is
> not a good idea.
> 
> I think right approach to abstract shadow entry in radix_tree is
> to mask off __GFP_ZERO in radix_tree's allocation APIs.

I don't think this is something the radix tree should know about.
SLAB should be checking for it (the patch I posted earlier in this
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.

diff --git a/mm/filemap.c b/mm/filemap.c
index c2147682f4c3..a87a523eea8e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -785,7 +785,7 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
 	VM_BUG_ON_PAGE(!PageLocked(new), new);
 	VM_BUG_ON_PAGE(new->mapping, new);
 
-	error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
+	error = radix_tree_preload(gfp_mask & ~(__GFP_HIGHMEM | __GFP_ZERO));
 	if (!error) {
 		struct address_space *mapping = old->mapping;
 		void (*freepage)(struct page *);
@@ -841,7 +841,8 @@ static int __add_to_page_cache_locked(struct page *page,
 			return error;
 	}
 
-	error = radix_tree_maybe_preload(gfp_mask & ~__GFP_HIGHMEM);
+	error = radix_tree_maybe_preload(gfp_mask &
+			~(__GFP_HIGHMEM | __GFP_ZERO));
 	if (error) {
 		if (!huge)
 			mem_cgroup_cancel_charge(page, memcg, false);