On Tue, Mar 12, 2013 at 03:32:21PM -0700, Andrew Morton wrote: > On Fri, 08 Mar 2013 20:37:36 +0800 Shuge <shugelinux@xxxxxxxxx> wrote: > > > The bounce accept slab pages from jbd2, and flush dcache on them. > > When enabling VM_DEBUG, it will tigger VM_BUG_ON in page_mapping(). > > So, check PageSlab to avoid it in __blk_queue_bounce(). > > > > Bug URL: http://lkml.org/lkml/2013/3/7/56 > > > > ... > > > > --- a/mm/bounce.c > > +++ b/mm/bounce.c > > @@ -214,7 +214,8 @@ static void __blk_queue_bounce(struct request_queue > > *q, struct bio **bio_orig, > > if (rw == WRITE) { > > char *vto, *vfrom; > > - flush_dcache_page(from->bv_page); > > + if (unlikely(!PageSlab(from->bv_page))) > > + flush_dcache_page(from->bv_page); > > vto = page_address(to->bv_page) + to->bv_offset; > > vfrom = kmap(from->bv_page) + from->bv_offset; > > memcpy(vto, vfrom, to->bv_len); > > I guess this is triggered by Catalin's f1a0c4aa0937975b ("arm64: Cache > maintenance routines"), which added a page_mapping() call to arm64's > arch/arm64/mm/flush.c:flush_dcache_page(). > > What's happening is that jbd2 is using kmalloc() to allocate buffer_head > data. That gets submitted down the BIO layer and __blk_queue_bounce() > calls flush_dcache_page() which in the arm64 case calls page_mapping() > and page_mapping() does VM_BUG_ON(PageSlab) and splat. > > The unusual thing about all of this is that the payload for some disk > IO is coming from kmalloc, rather than being a user page. It's oddball > but we've done this for ages and should continue to support it. > > > Now, the page from kmalloc() cannot be in highmem, so why did the > bounce code decide to bounce it? > > __blk_queue_bounce() does > > /* > * is destination page below bounce pfn? > */ > if (page_to_pfn(page) <= queue_bounce_pfn(q) && !force) > continue; > > and `force' comes from must_snapshot_stable_pages(). But > must_snapshot_stable_pages() must have returned false, because if it > had returned true then it would have been must_snapshot_stable_pages() > which went BUG, because must_snapshot_stable_pages() calls page_mapping(). > > So my tentative diagosis is that arm64 is fishy. A page which was > allocated via jbd2_alloc(GFP_NOFS)->kmem_cache_alloc() ended up being > above arm64's queue_bounce_pfn(). Can you please do a bit of > investigation to work out if this is what is happening? Find out why > __blk_queue_bounce() decided to bounce a page which shouldn't have been > bounced? That sure is strange. I didn't see any obvious reasons why we'd end up with a kmalloc above queue_bounce_pfn(). But then I don't have any arm64s either. > This is all terribly fragile :( afaict if someone sets > bdi_cap_stable_pages_required() against that jbd2 queue, we're going to > hit that BUG_ON() again, via must_snapshot_stable_pages()'s > page_mapping() call. (Darrick, this means you ;)) Wheeee. You're right, we shouldn't be calling page_mapping on slab pages. We can keep walking the bio segments to find a non-slab page that can tell us MS_SNAP_STABLE is set, since we probably won't need the bounce buffer anyway. How does something like this look? (+ the patch above) --D Subject: [PATCH] mm: Don't blow up on slab pages being written to disk Don't assume that all pages attached to a bio are non-slab pages. This happens if (for example) jbd2 allocates a buffer out of the slab to hold frozen data. If we encounter a slab page, just ignore the page and keep searching. Hopefully filesystems are smart enough to guarantee that slab pages won't be dirtied while they're also being written to disk. Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> --- mm/bounce.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/bounce.c b/mm/bounce.c index 5f89017..af34855 100644 --- a/mm/bounce.c +++ b/mm/bounce.c @@ -199,6 +199,8 @@ static int must_snapshot_stable_pages(struct request_queue *q, struct bio *bio) */ bio_for_each_segment(from, bio, i) { page = from->bv_page; + if (PageSlab(page)) + continue; mapping = page_mapping(page); if (!mapping) continue; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>