Messages in this thread | | | Subject | Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap() | From | Christian König <> | Date | Thu, 17 Sep 2020 09:11:33 +0200 |
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Am 17.09.20 um 08:23 schrieb Daniel Vetter: > On Wed, Sep 16, 2020 at 5:31 PM Christian König > <ckoenig.leichtzumerken@gmail.com> wrote: >> Am 16.09.20 um 17:24 schrieb Daniel Vetter: >>> On Wed, Sep 16, 2020 at 4:14 PM Christian König >>> <christian.koenig@amd.com> wrote: >>>> Am 16.09.20 um 16:07 schrieb Jason Gunthorpe: >>>>> On Wed, Sep 16, 2020 at 11:53:59AM +0200, Daniel Vetter wrote: >>>>> >>>>>> But within the driver, we generally need thousands of these, and that >>>>>> tends to bring fd exhaustion problems with it. That's why all the private >>>>>> buffer objects which aren't shared with other process or other drivers are >>>>>> handles only valid for a specific fd instance of the drm chardev (each >>>>>> open gets their own namespace), and only for ioctls done on that chardev. >>>>>> And for mmap we assign fake (but unique across all open fd on it) offsets >>>>>> within the overall chardev. Hence all the pgoff mangling and re-mangling. >>>>> Are they still unique struct files? Just without a fdno? >>>> Yes, exactly. >>> Not entirely, since dma-buf happened after drm chardev, so for that >>> historical reason the underlying struct file is shared, since it's the >>> drm chardev. But since that's per-device we don't have a problem in >>> practice with different vm_ops, since those are also per-device. But >>> yeah we could fish out some entirely hidden per-object struct file if >>> that's required for some mm internal reasons. >> Hui? Ok that is just the handling in i915, isn't it? >> >> As far as I know we create an unique struct file for each DMA-buf. > Yes dma-buf, but that gets forwarded to the original drm chardev which > originally exported the buffer. It's only there where the forwarding > chain stops. The other thing is that iirc we have a singleton > anon_inode behind all the dma-buf, so they'd share all the same > address_space and so would all alias for unmap_mapping_range (I think > at least).
Amdgpu works by using the address_space of the drm chardev into the struct file of DMA-buf instead.
I think that this is cleaner, but only by a little bit :)
Anyway I'm a bit concerned that we have so many different approaches for the same problem.
Christian.
> -Daniel > >> Regards, >> Christian. >> >> >>> -Daniel >>> >>>>>> Hence why we'd like to be able to forward aliasing mappings and adjust the >>>>>> file and pgoff, while hopefully everything keeps working. I thought this >>>>>> would work, but Christian noticed it doesn't really. >>>>> It seems reasonable to me that the dma buf should be the owner of the >>>>> VMA, otherwise like you say, there is a big mess attaching the custom >>>>> vma ops and what not to the proper dma buf. >>>>> >>>>> I don't see anything obviously against this in mmap_region() - why did >>>>> Chritian notice it doesn't really work? >>>> To clarify I think this might work. >>>> >>>> I just had the same "Is that legal?", "What about security?", etc.. >>>> questions you raised as well. >>>> >>>> It seems like a source of trouble so I thought better ask somebody more >>>> familiar with that. >>>> >>>> Christian. >>>> >>>>> Jason >>>> _______________________________________________ >>>> dri-devel mailing list >>>> dri-devel@lists.freedesktop.org >>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Fmailman%2Flistinfo%2Fdri-devel&data=02%7C01%7Cchristian.koenig%40amd.com%7Cf725d2eb6a5a49bd533f08d85ad23308%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637359206142262941&sdata=qcLsl9R1gP%2FGY39ctsQkIzI99Bn%2F840YS17F4xudrAE%3D&reserved=0 >>> >
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