linux-kernel - Re: [PATCH 1/1] dma-buf: heaps: Map system heap pages as managed by linux vm

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Date:   Tue, 2 Feb 2021 00:44:44 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "(Exiting) Benjamin Gaignard" <benjamin.gaignard@...aro.org>,
        Liam Mark <lmark@...eaurora.org>, labbott@...hat.com,
        Brian Starkey <Brian.Starkey@....com>,
        John Stultz <john.stultz@...aro.org>,
        "Christian K??nig" <christian.koenig@....com>,
        Chris Goldsworthy <cgoldswo@...eaurora.org>,
        "??rjan Eide" <orjan.eide@....com>,
        Robin Murphy <robin.murphy@....com>,
        James Jones <jajones@...dia.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        linux-media <linux-media@...r.kernel.org>,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] dma-buf: heaps: Map system heap pages as managed by
 linux vm

On Mon, Feb 1, 2021 at 11:03 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> IMHO the
>
>         BUG_ON(vma->vm_flags & VM_PFNMAP);
>
> in vm_insert_page should just become a WARN_ON_ONCE with an error
> return, and then we just need to gradually fix up the callers that
> trigger it instead of coming up with workarounds like this.

For the existing vm_insert_page users this should be fine since
BUG_ON() guarantees that none of them sets VM_PFNMAP. However, for the
system_heap_mmap I have one concern. When vm_insert_page returns an
error due to VM_PFNMAP flag, the whole mmap operation should fail
(system_heap_mmap returning an error leading to dma_buf_mmap failure).
Could there be cases when a heap user (DRM driver for example) would
be expected to work with a heap which requires VM_PFNMAP and at the
same time with another heap which requires !VM_PFNMAP? IOW, this
introduces a dependency between the heap and its
user. The user would have to know expectations of the heap it uses and
can't work with another heap that has the opposite expectation. This
usecase is purely theoretical and maybe I should not worry about it
for now?