Re: [Xen-devel] [PATCH] xen-swiotlb: exchange memory with Xen only when pages are contiguous
From: Joe Jin
Date: Tue Oct 30 2018 - 10:48:26 EST
On 10/30/18 7:21 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf
>> Of Joe Jin
>> Sent: 30 October 2018 14:13
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Boris Ostrovsky
>> <boris.ostrovsky@xxxxxxxxxx>; Konrad Rzeszutek Wilk
>> Cc: John Sobecki <john.sobecki@xxxxxxxxxx>; DONGLI.ZHANG
>> <dongli.zhang@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx" <linux-
>> kernel@xxxxxxxxxxxxxxx>; konrad@xxxxxxxxxx; xen-
>> devel@xxxxxxxxxxxxxxxxxxxx; Christoph Helwig <hch@xxxxxx>
>> Subject: Re: [Xen-devel] [PATCH] xen-swiotlb: exchange memory with Xen
>> only when pages are contiguous
>> 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
>>>> memory only allocated but was not freed, I guess it caused by driver
>>>> dma_pool, that means if lots of such requests, the list will consume
>>>> 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
>> Call trace of first called xen_swiotlb_alloc_coherent(The pages never
>> backed to Xen):
>> [ 23.436333] [<ffffffff814040c9>]
>> [ 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.
> Yes, it seems pools can hang onto a serious number of allocations so a list is probably not wise.
I agree with you.
> What I was pointing out, though, is that it appears you can't even track mappings (as opposed to allocations) with a list.
> either because drivers apparently try to unmap things they have not mapped.
If this happened, should be fixed by driver :)