[PATCH v2 2/2] dma-buf: heaps: Map system heap pages as managed by linux vm

[Suren Baghdasaryan]

On Tue, Feb 2, 2021 at 5:39 PM Minchan Kim <minchan at kernel.org> wrote:
>
> On Tue, Feb 02, 2021 at 04:31:34PM -0800, Suren Baghdasaryan wrote:
> > Currently system heap maps its buffers with VM_PFNMAP flag using
> > remap_pfn_range. This results in such buffers not being accounted
> > for in PSS calculations because vm treats this memory as having no
> > page structs. Without page structs there are no counters representing
> > how many processes are mapping a page and therefore PSS calculation
> > is impossible.
> > Historically, ION driver used to map its buffers as VM_PFNMAP areas
> > due to memory carveouts that did not have page structs [1]. That
> > is not the case anymore and it seems there was desire to move away
> > from remap_pfn_range [2].
> > Dmabuf system heap design inherits this ION behavior and maps its
> > pages using remap_pfn_range even though allocated pages are backed
> > by page structs.
> > Replace remap_pfn_range with vm_insert_page, following Laura's suggestion
> > in [1]. This would allow correct PSS calculation for dmabufs.
> >
> > [1] https://driverdev-devel.linuxdriverproject.narkive.com/v0fJGpaD/using-ion-memory-for-direct-io
> > [2] http://driverdev.linuxdriverproject.org/pipermail/driverdev-devel/2018-October/127519.html
> > (sorry, could not find lore links for these discussions)
> >
> > Suggested-by: Laura Abbott <labbott at kernel.org>
> > Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> Reviewed-by: Minchan Kim <minchan at kernel.org>
>
> A note: This patch makes dmabuf system heap accounted as PSS so
> if someone has relies on the size, they will see the bloat.
> IIRC, there was some debate whether PSS accounting for their
> buffer is correct or not. If it'd be a problem, we need to
> discuss how to solve it(maybe, vma->vm_flags and reintroduce
> remap_pfn_range for them to be respected).

I did not see debates about not including *mapped* dmabufs into PSS
calculation. I remember people were discussing how to account dmabufs
referred only by the FD but that is a different discussion. If the
buffer is mapped into the address space of a process then IMHO
including it into PSS of that process is not controversial.