On Thu 14-03-13 15:42:43, Darrick J. Wong wrote: > On Wed, Mar 13, 2013 at 10:02:16PM +0100, Jan Kara wrote: > > On Wed 13-03-13 12:44:29, Darrick J. Wong wrote: > > > On Wed, Mar 13, 2013 at 09:50:21AM +0100, Jan Kara wrote: > > > > On Tue 12-03-13 18:10:20, Darrick J. Wong wrote: > > > > > 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) > > > > Umm, this won't quite work. We can have a bio which has just PageSlab > > > > page attached and so you won't be able to get to the superblock. Heh, isn't > > > > the whole page_mapping() thing in must_snapshot_stable_pages() wrong? When we > > > > do direct IO, these pages come directly from userspace and hell knows where > > > > they come from. Definitely their page_mapping() doesn't give us anything > > > > useful... Sorry for not realizing this earlier when reviewing the patch. > > > > > > > > ... remembering why we need to get to sb and why ext3 needs this ... So > > > > maybe a better solution would be to have a bio flag meaning that pages need > > > > bouncing? And we would set it from filesystems that need it - in case of > > > > ext3 only writeback of data from kjournald actually needs to bounce the > > > > pages. Thoughts? > > > > > > What about dirty pages that don't result in journal transactions? I think > > > ext3_sync_file() eventually calls ext3_ordered_writepage, which then calls > > > __block_write_full_page, which in turn calls submit_bh(). > > So here we have two options: > > Either we let ext3 wait the same way as other filesystems when stable pages > > are required. Then only data IO from kjournald needs to be bounced (all > > other IO is properly protected by PageWriteback bit). > > > > Or we won't let ext3 wait (as it is now), keep the superblock flag that fs > > needs bouncing, and set the bio flag in __block_write_full_page() and > > kjournald based on the sb flag. > > > > I think the first option is slightly better but I don't feel strongly > > about that. > > I like that first option -- it contains the kludgery to jbd instead of > spreading it around. Here's a patch that passes a quick smoke test on ext[34], > xfs, and vfat. What do you think of this one? Should I create a > submit_snapshot_bh() instead of letting callers stuff in arbitrary dangerous > BH_ flags? Thanks for writing the patch. I think _submit_bh() is OK as you did it. I have just two comments below. > --- > From: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > Subject: [PATCH] mm: Make snapshotting pages for stable writes a per-bio operation > > Walking a bio's page mappings has proved problematic, so create a new bio flag > to indicate that a bio's data needs to be snapshotted in order to guarantee > stable pages during writeback. Next, for the one user (ext3/jbd) of > snapshotting, hook all the places where writes can be initiated without > PG_writeback set, and set BIO_SNAP_STABLE there. Finally, the MS_SNAP_STABLE > mount flag (only used by ext3) is now superfluous, so get rid of it. > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > --- > fs/buffer.c | 9 ++++++++- > fs/ext3/super.c | 1 - > fs/jbd/commit.c | 4 ++-- > include/linux/blk_types.h | 3 ++- > include/linux/buffer_head.h | 1 + > include/uapi/linux/fs.h | 1 - > mm/bounce.c | 21 +-------------------- > mm/page-writeback.c | 4 ---- > 8 files changed, 14 insertions(+), 30 deletions(-) > ... > diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c > index 86b39b1..b91b688 100644 > --- a/fs/jbd/commit.c > +++ b/fs/jbd/commit.c > @@ -163,7 +163,7 @@ static void journal_do_submit_data(struct buffer_head **wbuf, int bufs, > for (i = 0; i < bufs; i++) { > wbuf[i]->b_end_io = end_buffer_write_sync; > /* We use-up our safety reference in submit_bh() */ > - submit_bh(write_op, wbuf[i]); > + _submit_bh(write_op, wbuf[i], 1 << BIO_SNAP_STABLE); Please add a comment here why we need BIO_SNAP_STABLE. Something like: /* * Here we write back pagecache data that may be mmaped. Since we cannot * afford to clean the page and set PageWriteback here due to lock ordering * (page lock ranks above transaction start), the data can change while IO is * in flight. Tell the block layer it should bounce the bio pages if stable * data during write is required. */ > } > } > > @@ -667,7 +667,7 @@ start_journal_io: > clear_buffer_dirty(bh); > set_buffer_uptodate(bh); > bh->b_end_io = journal_end_buffer_io_sync; > - submit_bh(write_op, bh); > + _submit_bh(write_op, bh, 1 << BIO_SNAP_STABLE); And this isn't needed. Here we write out only metadata and JBD already handles copying those / waiting for IO in flight for metadata. The rest of the patch looks OK and I like it much more than the previous version :) Honza > } > cond_resched(); > > diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h > index cdf1119..22990cf 100644 > --- a/include/linux/blk_types.h > +++ b/include/linux/blk_types.h > @@ -111,12 +111,13 @@ struct bio { > #define BIO_FS_INTEGRITY 9 /* fs owns integrity data, not block layer */ > #define BIO_QUIET 10 /* Make BIO Quiet */ > #define BIO_MAPPED_INTEGRITY 11/* integrity metadata has been remapped */ > +#define BIO_SNAP_STABLE 12 /* bio data must be snapshotted during write */ > > /* > * Flags starting here get preserved by bio_reset() - this includes > * BIO_POOL_IDX() > */ > -#define BIO_RESET_BITS 12 > +#define BIO_RESET_BITS 13 > > #define bio_flagged(bio, flag) ((bio)->bi_flags & (1 << (flag))) > > diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h > index 5afc4f9..714d5d9 100644 > --- a/include/linux/buffer_head.h > +++ b/include/linux/buffer_head.h > @@ -181,6 +181,7 @@ void ll_rw_block(int, int, struct buffer_head * bh[]); > int sync_dirty_buffer(struct buffer_head *bh); > int __sync_dirty_buffer(struct buffer_head *bh, int rw); > void write_dirty_buffer(struct buffer_head *bh, int rw); > +int _submit_bh(int, struct buffer_head *, unsigned long); > int submit_bh(int, struct buffer_head *); > void write_boundary_block(struct block_device *bdev, > sector_t bblock, unsigned blocksize); > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h > index c7fc1e6..a4ed56c 100644 > --- a/include/uapi/linux/fs.h > +++ b/include/uapi/linux/fs.h > @@ -88,7 +88,6 @@ struct inodes_stat_t { > #define MS_STRICTATIME (1<<24) /* Always perform atime updates */ > > /* These sb flags are internal to the kernel */ > -#define MS_SNAP_STABLE (1<<27) /* Snapshot pages during writeback, if needed */ > #define MS_NOSEC (1<<28) > #define MS_BORN (1<<29) > #define MS_ACTIVE (1<<30) > diff --git a/mm/bounce.c b/mm/bounce.c > index 5f89017..a5c2ec3 100644 > --- a/mm/bounce.c > +++ b/mm/bounce.c > @@ -181,32 +181,13 @@ static void bounce_end_io_read_isa(struct bio *bio, int err) > #ifdef CONFIG_NEED_BOUNCE_POOL > static int must_snapshot_stable_pages(struct request_queue *q, struct bio *bio) > { > - struct page *page; > - struct backing_dev_info *bdi; > - struct address_space *mapping; > - struct bio_vec *from; > - int i; > - > if (bio_data_dir(bio) != WRITE) > return 0; > > if (!bdi_cap_stable_pages_required(&q->backing_dev_info)) > return 0; > > - /* > - * Based on the first page that has a valid mapping, decide whether or > - * not we have to employ bounce buffering to guarantee stable pages. > - */ > - bio_for_each_segment(from, bio, i) { > - page = from->bv_page; > - mapping = page_mapping(page); > - if (!mapping) > - continue; > - bdi = mapping->backing_dev_info; > - return mapping->host->i_sb->s_flags & MS_SNAP_STABLE; > - } > - > - return 0; > + return test_bit(BIO_SNAP_STABLE, &bio->bi_flags); > } > #else > static int must_snapshot_stable_pages(struct request_queue *q, struct bio *bio) > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index efe6814..4514ad7 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -2311,10 +2311,6 @@ void wait_for_stable_page(struct page *page) > > if (!bdi_cap_stable_pages_required(bdi)) > return; > -#ifdef CONFIG_NEED_BOUNCE_POOL > - if (mapping->host->i_sb->s_flags & MS_SNAP_STABLE) > - return; > -#endif /* CONFIG_NEED_BOUNCE_POOL */ > > wait_on_page_writeback(page); > } -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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