Re: [Xen-devel] [PATCH] xen-swiotlb: exchange memory with Xen only when pages are contiguous

[Joe Jin]

On 10/30/18 1:59 AM, Paul Durrant wrote:
>> On 10/25/18 11:56 AM, Joe Jin wrote:
>>> I just discussed this patch with Boris in private, his opinions(Boris,
>>> please correct me if any misunderstood) are:
>>>
>>> 1. With/without the check, both are incorrect, he thought we need to
>>>    prevented unalloc'd free at here.
>>> 2. On freeing, if upper layer already checked the memory was DMA-able,
>>>    the checking at here does not make sense, we can remove all checks.
>>> 3. xen_create_contiguous_region() and xen_destroy_contiguous_region()
>>>    to come in pairs.
>> I tried to added radix_tree to track allocating/freeing and I found some
>> memory only allocated but was not freed, I guess it caused by driver used
>> dma_pool, that means if lots of such requests, the list will consume lot
>> of memory for it. Will continue to work on it, if anyone have good idea
>> for it please let me know, I'd like to try it.
>>
> FWIW, in my Xen PV-IOMMU test patches, I have also tried keeping a list of ranges mapped for DMA and have discovered apparent issues with some drivers, particularly tg3, that seem to free mappings that have not been allocated (or possibly double-free). I've never fully tracked down the issue.

Call trace of first called xen_swiotlb_alloc_coherent(The pages never backed to Xen):

[   23.436333]  [<ffffffff814040c9>] xen_swiotlb_alloc_coherent+0x169/0x510
[   23.436623]  [<ffffffff811eb38d>] ? kmem_cache_alloc_trace+0x1ed/0x280
[   23.436900]  [<ffffffff811d72af>] dma_pool_alloc+0x11f/0x260
[   23.437190]  [<ffffffff81537442>] ehci_qh_alloc+0x52/0x120
[   23.437481]  [<ffffffff8153b80f>] ehci_setup+0x2bf/0x8e0
[   23.437760]  [<ffffffff81476d06>] ? __dev_printk+0x46/0xa0
[   23.438042]  [<ffffffff814770b3>] ? _dev_info+0x53/0x60
[   23.438327]  [<ffffffff8153f620>] ehci_pci_setup+0xc0/0x5f0
[   23.438615]  [<ffffffff81519fcd>] usb_add_hcd+0x25d/0xaf0
[   23.438901]  [<ffffffff8152c9a6>] usb_hcd_pci_probe+0x406/0x520
[   23.439177]  [<ffffffff8153f486>] ehci_pci_probe+0x36/0x40
[   23.439469]  [<ffffffff8136e99a>] local_pci_probe+0x4a/0xb0
[   23.439752]  [<ffffffff8136fba5>] ? pci_match_device+0xe5/0x110
[   23.440027]  [<ffffffff8136fce1>] pci_device_probe+0xd1/0x120
[   23.440320]  [<ffffffff8147b13c>] driver_probe_device+0x20c/0x4d0
[   23.440599]  [<ffffffff8147b4eb>] __driver_attach+0x9b/0xa0
[   23.440879]  [<ffffffff8147b450>] ? __device_attach+0x50/0x50

Above was EHCI used DMA pool to allocate DMA memory.

During my testing, ~1000 entries was not freed, if more PCI devices
use DMA pool, the tree/list will have more entries, looks it's not a
good idea that use a list to track it.

Thanks,
Joe