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SubjectRe: Fw: crash on x86_64 - mm related?
On Wed, 18 Jan 2006, Hugh Dickins wrote:
> On Tue, 17 Jan 2006, Ryan Richter wrote:
>
> > This machine experienced another random reboot today, nothing in the
> > logs or on the console etc. This is the 3rd time now since I upgraded
> > from 2.6.11.3. Is there any way to debug something like this?
>
> Nasty. I hope someone else can suggest something constructive.

Are they still happening?

> > I'm fairly certain it's not hardware-related.
> > Might this have something to do with the st problem?
>
> Well, it might: I've not been able to explain that at all, so cannot
> rule out a relation to your reboots.

If no "Bad page state" has occurred earlier, we can now be sure that
these reboots are (unfortunately) unrelated to the st problem.

> The st problem (Bad page state,
> mapcount 2 while count 0, from sgl_unmap_user_pages) ought to be a lot
> easier to debug than a random reboot: but I've still no suggestion.

I guessed it last week, Ryan verified that booting with "iommu=nomerge"
worked around it, and then that the 2.6.15 patch below fixes it.

On some architectures (alpha, ia64, parisc, powerpc, sparc64, x86_64,
more?) in some configurations, dma_map_sg or pci_map_sg may coalesce
scatterlist entries, copying later entries down: bad news if we then
go on to use that coalesced scatterlist to free the pages afterwards -
we're quite liable to free the same page twice.

James' DMA-API.txt is perfectly clear that "The mapping process is
allowed to destroy information in the sg". I wonder that this hasn't
been noticed before. Perhaps we should have a debug option to destroy
everything in the scatterlist when unmapping.

The st code has moved on a little since 2.6.15, I'll send the 2.6.16-rc2
patch to Kai and lists in reply to this mail. I've looked around for
similar culprits - fewer than I was expecting, but I'm not sure if
I've found them all:

drivers/infiniband/core/uverbs_mem.c
drivers/scsi/ipr.c
drivers/scsi/osst.c
drivers/scsi/sg.c

I'll reply with _untested_ patches to the maintainers and list for
those too. Except for sg.c, which I'll send privately to Doug -
I've wasted a lot of time not quite understanding it, and don't
want to expose my ignorance in public.

Hugh

--- 2.6.15/drivers/scsi/st.c 2006-01-03 03:21:10.000000000 +0000
+++ linux/drivers/scsi/st.c 2006-01-29 17:21:47.000000000 +0000
@@ -188,11 +188,11 @@ static int from_buffer(struct st_buffer
static void move_buffer_data(struct st_buffer *, int);
static void buf_to_sg(struct st_buffer *, unsigned int);

-static int st_map_user_pages(struct scatterlist *, const unsigned int,
+static int st_map_user_pages(struct st_buffer *, const unsigned int,
unsigned long, size_t, int, unsigned long);
-static int sgl_map_user_pages(struct scatterlist *, const unsigned int,
+static int sgl_map_user_pages(struct st_buffer *, const unsigned int,
unsigned long, size_t, int);
-static int sgl_unmap_user_pages(struct scatterlist *, const unsigned int, int);
+static int sgl_unmap_user_pages(struct st_buffer *, const unsigned int, int);

static int st_probe(struct device *);
static int st_remove(struct device *);
@@ -1402,7 +1402,7 @@ static int setup_buffering(struct scsi_t
i = STp->try_dio && try_wdio;
if (i && ((unsigned long)buf & queue_dma_alignment(
STp->device->request_queue)) == 0) {
- i = st_map_user_pages(&(STbp->sg[0]), STbp->use_sg,
+ i = st_map_user_pages(STbp, STbp->use_sg,
(unsigned long)buf, count, (is_read ? READ : WRITE),
STp->max_pfn);
if (i > 0) {
@@ -1455,7 +1455,7 @@ static void release_buffering(struct scs

STbp = STp->buffer;
if (STbp->do_dio) {
- sgl_unmap_user_pages(&(STbp->sg[0]), STbp->do_dio, 0);
+ sgl_unmap_user_pages(STbp, STbp->do_dio, 0);
STbp->do_dio = 0;
}
}
@@ -4396,32 +4396,33 @@ static void do_create_class_files(struct
- any page is above max_pfn
(i.e., either completely successful or fails)
*/
-static int st_map_user_pages(struct scatterlist *sgl, const unsigned int max_pages,
+static int st_map_user_pages(struct st_buffer *STbp, const unsigned int max_pages,
unsigned long uaddr, size_t count, int rw,
unsigned long max_pfn)
{
int i, nr_pages;

- nr_pages = sgl_map_user_pages(sgl, max_pages, uaddr, count, rw);
+ nr_pages = sgl_map_user_pages(STbp, max_pages, uaddr, count, rw);
if (nr_pages <= 0)
return nr_pages;

for (i=0; i < nr_pages; i++) {
- if (page_to_pfn(sgl[i].page) > max_pfn)
+ if (page_to_pfn(STbp->sg[i].page) > max_pfn)
goto out_unmap;
}
return nr_pages;

out_unmap:
- sgl_unmap_user_pages(sgl, nr_pages, 0);
+ sgl_unmap_user_pages(STbp, nr_pages, 0);
return 0;
}


/* The following functions may be useful for a larger audience. */
-static int sgl_map_user_pages(struct scatterlist *sgl, const unsigned int max_pages,
+static int sgl_map_user_pages(struct st_buffer *STbp, const unsigned int max_pages,
unsigned long uaddr, size_t count, int rw)
{
+ struct scatterlist *sgl = STbp->sg;
unsigned long end = (uaddr + count + PAGE_SIZE - 1) >> PAGE_SHIFT;
unsigned long start = uaddr >> PAGE_SHIFT;
const int nr_pages = end - start;
@@ -4485,7 +4486,7 @@ static int sgl_map_user_pages(struct sca
sgl[0].length = count;
}

- kfree(pages);
+ STbp->sg_pages = pages;
return nr_pages;

out_unmap:
@@ -4500,13 +4501,13 @@ static int sgl_map_user_pages(struct sca


/* And unmap them... */
-static int sgl_unmap_user_pages(struct scatterlist *sgl, const unsigned int nr_pages,
+static int sgl_unmap_user_pages(struct st_buffer *STbp, const unsigned int nr_pages,
int dirtied)
{
int i;

for (i=0; i < nr_pages; i++) {
- struct page *page = sgl[i].page;
+ struct page *page = STbp->sg_pages[i];

if (dirtied)
SetPageDirty(page);
@@ -4516,5 +4517,7 @@ static int sgl_unmap_user_pages(struct s
page_cache_release(page);
}

+ kfree(STbp->sg_pages);
+ STbp->sg_pages = NULL;
return 0;
}
--- 2.6.15/drivers/scsi/st.h 2005-10-28 01:02:08.000000000 +0100
+++ linux/drivers/scsi/st.h 2006-01-29 16:52:02.000000000 +0000
@@ -37,6 +37,7 @@ struct st_buffer {
unsigned short frp_segs; /* number of buffer segments */
unsigned int frp_sg_current; /* driver buffer length currently in s/g list */
struct st_buf_fragment *frp; /* the allocated buffer fragment list */
+ struct page **sg_pages; /* pages to be freed from s/g list */
struct scatterlist sg[1]; /* MUST BE last item */
};

-
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