On Tue, 23 Oct 2007 09:09:33 +0200
Jens Axboe <jens.axboe@oracle.com> wrote:

quoted text
> On Tue, Oct 23 2007, Jens Axboe wrote:
> > On Mon, Oct 22 2007, David Miller wrote:
> > > 
> > > I'm debugging a blk_rq_map_sg() crash that i'm getting on sparc64 as
> > > root is mounted over IDE.  I think I know what is happening now.
> > > 
> > > The IDE sg table is allocated and initialized like this in
> > > drivers/ide/ide-probe.c:
> > > 
> > > 	x = kmalloc(sizeof(struct scatterlist) * nents, GFP_XXX);
> > > 	sg_init_table(x, nents);
> > > 
> > > So far, so good.
> > > 
> > > Now, ide_map_sg() passes requests down to blk_rq_map_sg() like this in
> > > drivers/block/ide-io.c:
> > > 
> > > 		hwif->sg_nents = blk_rq_map_sg(drive->queue, rq, sg);
> > > 
> > > Ok, so what does blk_rq_map_sg() do?
> > > 
> > > 	sg = NULL;
> > > 	rq_for_each_segment(bvec, rq, iter) {
> > >  ...
> > > 		if (bvprv && cluster) {
> > >  ...
> > > 		} else {
> > > new_segment:
> > > 			if (!sg)
> > > 				sg = sglist;
> > > 			else
> > > 				sg = sg_next(sg);
> > >  ...
> > > 		}
> > > 		bvprv = bvec;
> > > 	} /* segments in rq */
> > > 
> > > 	if (sg)
> > > 		__sg_mark_end(sg);
> > > 
> > > So let's say the first request comes in and needs 2 segs.
> > > This will mark sg[1].page_link with 0x2
> > > 
> > > If the next request from IDE needs 4 segs, we'll OOPS because
> > > sg_next() on &sg[1] will see page_link bit 0x2 is set and
> > > therefore return NULL.
> > > 
> > > A quick look shows that if you're testing on SCSI (or something
> > > layered on top of it like SATA or PATA) you won't see this seemingly
> > > guarenteed crash because the SCSI mid-layer allocates a fresh sglist
> > > via mempool_alloc() and runs sg_init_table() on it for every I/O
> > > request.
> > 
> > We should never see the end pointer in blk_rq_map_sg(), or that's a bug
> > in the driver. So it should be OK to just clear the end pointer always
> > in there, even if it's not the prettiest solution...
> > 
> > This just needs to be wrapped up in some scatterlist.h macro/function.
> > 
> > diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> > index 61c2e39..a3bda2f 100644
> > --- a/block/ll_rw_blk.c
> > +++ b/block/ll_rw_blk.c
> > @@ -1354,6 +1354,12 @@ new_segment:
> >  			else
> >  				sg = sg_next(sg);
> >  
> > +			/*
> > +			 * Clear end-of-table pointer, we'll mark a new one
> > +			 * at the end
> > +			 */
> > +			sg->page_link &= ~0x2;
> > +
> >  			sg_dma_len(sg) = 0;
> >  			sg_dma_address(sg) = 0;
> >  			sg_set_page(sg, bvec->bv_page);
> 
> Eh this wont work, it's the wrong entry... Here's a temporary
> work-around.

Yeah, it won't work. Now we must call sg_init_table for every I/O
request (it's not nice).

I think that there are other blk_rq_map_sg users need such fix.


quoted text
> diff --git a/drivers/ide/ide-io.c b/drivers/ide/ide-io.c
> index c89f0d3..108202b 100644
> --- a/drivers/ide/ide-io.c
> +++ b/drivers/ide/ide-io.c
> @@ -822,6 +822,7 @@ void ide_map_sg(ide_drive_t *drive, struct request *rq)
>  		return;
>  
>  	if (rq->cmd_type != REQ_TYPE_ATA_TASKFILE) {
> +		sg_init_table(hwif->sg_table, hwif->sg_max_nents);
>  		hwif->sg_nents = blk_rq_map_sg(drive->queue, rq, sg);
>  	} else {
>  		sg_init_one(sg, rq->buffer, rq->nr_sectors * SECTOR_SIZE);
> diff --git a/drivers/ide/ide-probe.c b/drivers/ide/ide-probe.c
> index ec55a17..cbfb113 100644
> --- a/drivers/ide/ide-probe.c
> +++ b/drivers/ide/ide-probe.c
> @@ -1324,8 +1324,6 @@ static int hwif_init(ide_hwif_t *hwif)
>  		goto out;
>  	}
>  
> -	sg_init_table(hwif->sg_table, hwif->sg_max_nents);
> -	
>  	if (init_irq(hwif) == 0)
>  		goto done;
>  
> 
> -- 
> Jens Axboe
> 
-
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