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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----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
> <konrad.wilk@xxxxxxxxxx>
> 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
> 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.
> 

Yes, it seems pools can hang onto a serious number of allocations so a list is 
probably not wise. 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.

  Paul

> Thanks,
> Joe
> 
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