Re: 5.3-rc1 regression with XFS log recovery

[Ming Lei <ming.lei@redhat.com>]

On Tue, Aug 20, 2019 at 04:13:26PM +0800, Ming Lei wrote:
> On Tue, Aug 20, 2019 at 07:53:20AM +0200, hch@lst.de wrote:
> > On Tue, Aug 20, 2019 at 02:41:35PM +1000, Dave Chinner wrote:
> > > > With the following debug patch.  Based on that I think I'll just
> > > > formally submit the vmalloc switch as we're at -rc5, and then we
> > > > can restart the unaligned slub allocation drama..
> > > 
> > > This still doesn't make sense to me, because the pmem and brd code
> > > have no aligment limitations in their make_request code - they can
> > > handle byte adressing and should not have any problem at all with
> > > 8 byte aligned memory in bios.
> > > 
> > > Digging a little furhter, I note that both brd and pmem use
> > > identical mechanisms to marshall data in and out of bios, so they
> > > are likely to have the same issue.
> > > 
> > > So, brd_make_request() does:
> > > 
> > >         bio_for_each_segment(bvec, bio, iter) {
> > >                 unsigned int len = bvec.bv_len;
> > >                 int err;
> > > 
> > >                 err = brd_do_bvec(brd, bvec.bv_page, len, bvec.bv_offset,
> > >                                   bio_op(bio), sector);
> > >                 if (err)
> > >                         goto io_error;
> > >                 sector += len >> SECTOR_SHIFT;
> > >         }
> > > 
> > > So, the code behind bio_for_each_segment() splits multi-page bvecs
> > > into individual pages, which are passed to brd_do_bvec(). An
> > > unaligned 4kB io traces out as:
> > > 
> > >  [  121.295550] p,o,l,s 00000000a77f0146,768,3328,0x7d0048
> > >  [  121.297635] p,o,l,s 000000006ceca91e,0,768,0x7d004e
> > > 
> > > i.e. page		offset	len	sector
> > > 00000000a77f0146	768	3328	0x7d0048
> > > 000000006ceca91e	0	768	0x7d004e
> > > 
> > > You should be able to guess what the problems are from this.
> 
> The problem should be that offset of '768' is passed to bio_add_page().

It can be quite hard to deal with non-512 aligned sector buffer, since
one sector buffer may cross two pages, so far one workaround I thought
of is to not merge such IO buffer into one bvec.

Verma, could you try the following patch?

diff --git a/block/bio.c b/block/bio.c
index 24a496f5d2e2..49deab2ac8c4 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -769,6 +769,9 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return false;
 
+	if (off & 511)
+		return false;
+
 	if (bio->bi_vcnt > 0) {
 		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
 

Thanks,
Ming