Re: [i915] b12d691ea5: kernel_BUG_at_mm/memory.c

[Kalle Valo]

Christoph Hellwig <hch@xxxxxx> writes:

> On Tue, May 18, 2021 at 04:58:31PM -1000, Linus Torvalds wrote:
>> On Tue, May 18, 2021 at 4:26 PM kernel test robot <oliver.sang@xxxxxxxxx> wrote:
>> >
>> > commit: b12d691ea5e01db42ccf3b4207e57cb3ce7cfe91 ("i915: fix remap_io_sg to verify the pgprot")
>> > [...]
>> > [  778.550996] kernel BUG at mm/memory.c:2183!
>> > [ 778.559015] RIP: 0010:remap_pfn_range_notrack
>> > (kbuild/src/consumer/mm/memory.c:2183
>> > kbuild/src/consumer/mm/memory.c:2211
>> > kbuild/src/consumer/mm/memory.c:2233
>> > kbuild/src/consumer/mm/memory.c:2255
>> > kbuild/src/consumer/mm/memory.c:2311)
>> > [  778.688951] remap_pfn_range (kbuild/src/consumer/mm/memory.c:2342)
>> > [  778.692700] remap_io_sg (kbuild/src/consumer/drivers/gpu/drm/i915/i915_mm.c:71) i915
>> 
>> Yeah, so that BUG_ON() checks that theer isn't any old mapping there.
>> 
>> You can't just remap over an old one, but it does seem like that is
>> exactly what commit b12d691ea5e0 ("i915: fix remap_io_sg to verify the
>> pgprot") ends up doing.
>> 
>> So the code used to just do "apply_to_page_range()", which admittedly
>> was odd too. But it didn't mind having old mappings and re-applying
>> something over them.
>> 
>> Converting it to use remap_pfn_range() does look better, but it kind
>> of depends on it ever being done *once*. But the caller seems to very
>> much remap the whole vmsa at fault time, so...
>> 
>> I don't know what the right thing to do here is, because I don't know
>> the invalidation logic and when faults happen.
>> 
>> I see that there is another thread about different issues on the
>> intel-gfx list. Adding a few people to this kernel test robot thread
>> too.
>> 
>> I'd be inclined to revert the commits as "not ready yet", but it would
>> be better if somebody can go "yeah, this should be done properly like
>> X".
>
> I think reverting just this commit for now is the best thing.

Yes, please revert it if there's no quick fix. On my Dell XPS 13 9310
laptop (with Debian 10) X won't start until I revert commit
b12d691ea5e0, so this is a major issue.

Also adding the new regressions list, as this is an i915 regression
introduced in v5.13-rc1.

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