Re: [PATCH] dma-debug: disable DMA_API_DEBUG for now

FUJITA Tomonori
2009-06-05T11:39:17Z

On Fri, 5 Jun 2009 12:41:33 +0200
Joerg Roedel <joerg.roedel@amd.com> wrote:

>
> On Fri, Jun 05, 2009 at 05:33:14PM +0900, FUJITA Tomonori wrote:
> > dma-debugs wrongly assumes that no multiple DMA transfers are
> > performed against the same dma address on one device at the same
> > time. However it's true only with hardware IOMMUs. For example, an
> > application can easily send the same buffer twice with different
> > lengths to one device by using DIO and AIO. If these requests are not
> > unmapped in the same order in which they were mapped,
> > hash_bucket_find() finds a wrong entry and gives a false warning.
> >
> > We should fix this before 2.6.30 release. Seems that there is no
> > easy way to fix it. I think that it's better to just disable
> > dma-debug for now.
> >
> > Torsten Kaiser found this bug with the RAID1 configuration:
> >
> > http://marc.info/?t=124336541900008&r=1&w=2
> >
>
> Argh, I never thought that this can happen. But its not explicitly
> forbidden so we have to handle this situation. Thanks for tracking down
> the bug to this issue.
> However, I think there is a somehow simple fix for the issue. Patch is
> attached. Its the least intrusive way I can think of to fix this
> problem.
> But its up to Linus/Ingo to decide if it can be accepted at this very
> late point in the cycle. Since dma-debug is new with 2.6.30 it will at
> least not introduce any regression. The patch is boot and basically
> load-tested with some disk and network load and showed no issues on my
> test machine. The diffstat looks big but more than the half of the patch
> adds comments to explain what it does. So the diffstat looks bigger than
> the actual change.
> Linus, if you think my patch is not acceptable at this point then please
> apply the patch from FUJITA Tomonori.
>
> Regards,
>
> Joerg
>
> From c0baa4d43a674c9cc321af3b23a6d5c9b46d65bd Mon Sep 17 00:00:00 2001
> From: Joerg Roedel <joerg.roedel@amd.com>
> Date: Fri, 5 Jun 2009 12:01:35 +0200
> Subject: [PATCH] dma-debug: change hash_bucket_find from first-fit to best-fit
>
> Some device drivers map the same physical address multiple times to a
> dma address. Without an IOMMU this results in the same dma address being
> put into the dma-debug hash multiple times. With a first-fit match in
> hash_bucket_find() this function may return the wrong dma_debug_entry.
> This can result in false positive warnings. This patch fixes it by
> changing the first-fit behavior of hash_bucket_find() into a best-fit
> algorithm.

I thought about something like this fix however we can do better; with
this fix, we could overlook a bad hardware IOMMU bug.

I think that the better fix can handle both cases per device:

- multiple identical dma addresses should not happen (with devices
behind hardware IOMMU)
- multiple identical dma addresses could happen

It's better to use a strict-fit algorithm (as we do now) with the
former. It's pretty difficult to do something better than a best-fit
algorithm with the latter though.

> Reported-by: Torsten Kaiser <just.for.lkml@googlemail.com>
> Reported-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
> ---
> lib/dma-debug.c | 43 +++++++++++++++++++++++++++++++++++++++----
> 1 files changed, 39 insertions(+), 4 deletions(-)
>
> diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> index 69da09a..2b16536 100644
> --- a/lib/dma-debug.c
> +++ b/lib/dma-debug.c
> @@ -185,15 +185,50 @@ static void put_hash_bucket(struct hash_bucket *bucket,
> static struct dma_debug_entry *hash_bucket_find(struct hash_bucket *bucket,
> struct dma_debug_entry *ref)
> {
> - struct dma_debug_entry *entry;
> + struct dma_debug_entry *entry, *ret = NULL;
> + int matches = 0, match_lvl, last_lvl = 0;
>
> list_for_each_entry(entry, &bucket->list, list) {
> - if ((entry->dev_addr == ref->dev_addr) &&
> - (entry->dev == ref->dev))
> + if ((entry->dev_addr != ref->dev_addr) ||
> + (entry->dev != ref->dev))
> + continue;
> +
> + /*
> + * Some drivers map the same physical address multiple
> + * times. Without a hardware IOMMU this results in the
> + * same device addresses being put into the dma-debug
> + * hash multiple times too. This can result in false
> + * positives being reported. Therfore we implement a
> + * best-fit algorithm here which returns the entry from
> + * the hash which fits best to the reference value
> + * instead of the first-fit.
> + */
> + matches += 1;
> + match_lvl = 0;
> + entry->size == ref->size ? ++match_lvl : match_lvl;
> + entry->type == ref->type ? ++match_lvl : match_lvl;
> + entry->direction == ref->direction ? ++match_lvl : match_lvl;
> +
> + if (match_lvl == 3) {
> + /* perfect-fit - return the result */
> return entry;
> + } else if (match_lvl > last_lvl) {
> + /*
> + * We found an entry that fits better then the
> + * previous one
> + */
> + last_lvl = match_lvl;
> + ret = entry;
> + }
> }
>
> - return NULL;
> + /*
> + * If we have multiple matches but no perfect-fit, just return
> + * NULL.
> + */
> + ret = (matches == 1) ? ret : NULL;
> +
> + return ret;
> }
>
> /*
> --
> 1.6.3.1
>
>
>
>
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