On Tue, Oct 23 2007, FUJITA Tomonori wrote:
> On Tue, 23 Oct 2007 09:09:33 +0200
> Jens Axboe <[email protected]> wrote:
>
> > 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 the fix would be to have a sg_next_and_clear() or something that
doesn't honor the 0x02 termination bit and clears it, for the cases
where you KNOW that there are more entries.
> I think that there are other blk_rq_map_sg users need such fix.
Possibly, I did do quite a few of them. Alternatively, we can remove
__sg_mark_end() and leave that up to the driver.
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 61c2e39..290836f 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1352,7 +1352,7 @@ new_segment:
if (!sg)
sg = sglist;
else
- sg = sg_next(sg);
+ sg = sg_next_force(sg);
sg_dma_len(sg) = 0;
sg_dma_address(sg) = 0;
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 42daf5e..a98a2ee 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -99,6 +99,22 @@ static inline struct scatterlist *sg_next(struct scatterlist *sg)
return sg;
}
+/**
+ * sg_next_force - return the next scatterlist entry in a list
+ * @sg: The current sg entry
+ *
+ * Description:
+ * Must only be used when more entries beyond this one is known to exist,
+ * as it clears the termination bit. Useful to avoid adding a full sg
+ * table init on every mapping.
+ *
+ **/
+static inline struct scatterlist *sg_next_force(struct scatterlist *sg)
+{
+ sg->page_link &= ~0x02;
+ return sg_next(sg);
+}
+
/*
* Loop over each sg element, following the pointer to a new list if necessary
*/
--
Jens Axboe
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