On Tue, Aug 20, 2019 at 04:13:26PM +0800, Ming Lei wrote:
> On Tue, Aug 20, 2019 at 07:53:20AM +0200, hch@xxxxxx 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
