linux-kernel - Re: [PATCH v1: vfio: avoid unnecessary pin memory when dma map io address space 0/2]

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Message-ID: <20241024110624.63871cfa.alex.williamson@redhat.com>
Date: Thu, 24 Oct 2024 11:06:24 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Qinyun Tan <qinyuntan@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1: vfio: avoid unnecessary pin memory when dma map io
 address space 0/2]

On Thu, 24 Oct 2024 17:34:42 +0800
Qinyun Tan <qinyuntan@...ux.alibaba.com> wrote:

> When user application call ioctl(VFIO_IOMMU_MAP_DMA) to map a dma address,
> the general handler 'vfio_pin_map_dma' attempts to pin the memory and
> then create the mapping in the iommu.
> 
> However, some mappings aren't backed by a struct page, for example an
> mmap'd MMIO range for our own or another device. In this scenario, a vma
> with flag VM_IO | VM_PFNMAP, the pin operation will fail. Moreover, the
> pin operation incurs a large overhead which will result in a longer
> startup time for the VM. We don't actually need a pin in this scenario.
> 
> To address this issue, we introduce a new DMA MAP flag
> 'VFIO_DMA_MAP_FLAG_MMIO_DONT_PIN' to skip the 'vfio_pin_pages_remote'
> operation in the DMA map process for mmio memory. Additionally, we add
> the 'VM_PGOFF_IS_PFN' flag for vfio_pci_mmap address, ensuring that we can
> directly obtain the pfn through vma->vm_pgoff.
> 
> This approach allows us to avoid unnecessary memory pinning operations,
> which would otherwise introduce additional overhead during DMA mapping.
> 
> In my tests, using vfio to pass through an 8-card AMD GPU which with a
> large bar size (128GB*8), the time mapping the 192GB*8 bar was reduced
> from about 50.79s to 1.57s.

If the vma has a flag to indicate pfnmap, why does the user need to
provide a mapping flag to indicate not to pin?  We generally cannot
trust such a user directive anyway, nor do we in this series, so it all
seems rather redundant.

What about simply improving the batching of pfnmap ranges rather than
imposing any sort of mm or uapi changes?  Or perhaps, since we're now
using huge_fault to populate the vma, maybe we can iterate at PMD or
PUD granularity rather than PAGE_SIZE?  Seems like we have plenty of
optimizations to pursue that could be done transparently to the user.
Thanks,

Alex