This bug could happen on alpha, parisc, and sparc, which use VMERGE.
Parisc doesn't use virtual merge accounting (there is variable for it but
it's always 0).
Hmm, really? Looks like PARISC IOMMUs (ccio-dma.c and sba_iomm.c) set
parisc_vmerge_boundary (CC'ed PARISC mailing list).
On sparc64 it is broken anyway with or without your patch.
Yeah, we need to modify SPARC64 IOMMU code (I'm not sure that it's
worth). Right now, the best fix is setting BIO_VMERGE_BOUNDARY to 0.
--And alpha alone doesn't justify substantial code bloat in generic block
layer. So I propose this patch to drop it at all.
Jens, what do you think about removing VMERGE code?