Re: kernel BUG at mm/huge_memory.c:2613!

Robin Murphy
2020-06-22T15:31:00Z

On 2020-06-22 13:46, Joerg Roedel wrote:
> + Robin
>
> Robin, any idea on this?

After a bit of archaeology, this dates back to the original review:

https://lore.kernel.org/linux-arm-kernel/54C285D4.3070802@arm.com/
https://lore.kernel.org/linux-arm-kernel/54DA2666.9030003@arm.com/

In summary: originally this inherited from other arch code that did
simply strip __GFP_COMP; that was deemed questionable because of the
nonsensical comment about CONFIG_HUGETLBFS that was stuck to it; the
current code is like it is because in 5 and a half years nobody said
that it's wrong :)

If there actually *are* good reasons for stripping __GFP_COMP, then I've
certainly no objection to doing so.

Robin.

> On Thu, Jun 18, 2020 at 10:40:26PM -0400, Andrea Arcangeli wrote:
>> Hello,
>>
>> On Thu, Jun 18, 2020 at 06:14:49PM -0700, Roman Gushchin wrote:
>>> I agree. The whole
>>>
>>> page = alloc_pages_node(nid, alloc_flags, order);
>>> if (!page)
>>> continue;
>>> if (!order)
>>> break;
>>> if (!PageCompound(page)) {
>>> split_page(page, order);
>>> break;
>>> } else if (!split_huge_page(page)) {
>>> break;
>>> }
>>>
>>> looks very suspicious to me.
>>> My wild guess is that gfp flags changed somewhere above, so we hit
>>> the branch which was never hit before.
>>
>> Right to be suspicious about the above: split_huge_page on a regular
>> page allocated by a driver was never meant to work.
>>
>> The PageLocked BUG_ON is just a symptom of a bigger issue, basically
>> split_huge_page it may survive, but it'll stay compound and in turn it
>> must be freed as compound.
>>
>> The respective free method doesn't even contemplate freeing compound
>> pages, the only way the free method can survive, is by removing
>> __GFP_COMP forcefully in the allocation that was perhaps set here
>> (there are that many __GFP_COMP in that directory):
>>
>> static void snd_malloc_dev_pages(struct snd_dma_buffer *dmab, size_t size)
>> {
>> gfp_t gfp_flags;
>>
>> gfp_flags = GFP_KERNEL
>> | __GFP_COMP /* compound page lets parts be mapped */
>>
>> And I'm not sure what the comment means here, compound or non compound
>> doesn't make a difference when you map it, it's not a THP, the
>> mappings must be handled manually so nothing should check PG_compound
>> anyway in the mapping code.
>>
>> Something like this may improve things, it's an untested quick hack,
>> but this assumes it's always a bug to setup a compound page for these
>> DMA allocations and given the API it's probably a correct
>> assumption.. Compound is slower, unless you need it, you can avoid it
>> and then split_page will give contiguous memory page granular. Ideally
>> the code shouldn't call split_page at all and it should free it all at
>> once by keeping track of the order and by returning the order to the
>> caller, something the API can't do right now as it returns a plain
>> array that can only represent individual small pages.
>>
>> Once this is resolved, you may want to check your config, iommu passthrough
>> sounds more optimal for a soundcard.
>>
>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>> index f68a62c3c32b..3dfbc010fa83 100644
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> @@ -499,6 +499,10 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
>>
>> /* IOMMU can map any pages, so himem can also be used here */
>> gfp |= __GFP_NOWARN | __GFP_HIGHMEM;
>> + if (unlikely(gfp & __GFP_COMP)) {
>> + WARN();
>> + gfp &= ~__GFP_COMP;
>> + }
>>
>> while (count) {
>> struct page *page = NULL;
>> @@ -522,13 +526,8 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
>> continue;
>> if (!order)
>> break;
>> - if (!PageCompound(page)) {
>> - split_page(page, order);
>> - break;
>> - } else if (!split_huge_page(page)) {
>> - break;
>> - }
>> - __free_pages(page, order);
>> + split_page(page, order);
>> + break;
>> }
>> if (!page) {
>> __iommu_dma_free_pages(pages, i);
>> diff --git a/sound/core/memalloc.c b/sound/core/memalloc.c
>> index 6850d13aa98c..378f5a36ec5f 100644
>> --- a/sound/core/memalloc.c
>> +++ b/sound/core/memalloc.c
>> @@ -28,7 +28,6 @@ static void snd_malloc_dev_pages(struct snd_dma_buffer *dmab, size_t size)
>> gfp_t gfp_flags;
>>
>> gfp_flags = GFP_KERNEL
>> - | __GFP_COMP /* compound page lets parts be mapped */
>> | __GFP_NORETRY /* don't trigger OOM-killer */
>> | __GFP_NOWARN; /* no stack trace print - this call is non-critical */
>> dmab->area = dma_alloc_coherent(dmab->dev.dev, size, &dmab->addr,