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

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Date:   Tue, 2 Feb 2021 17:39:21 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     sumit.semwal@...aro.org, akpm@...ux-foundation.org,
        hch@...radead.org, lmark@...eaurora.org, labbott@...hat.com,
        Brian.Starkey@....com, john.stultz@...aro.org,
        christian.koenig@....com, cgoldswo@...eaurora.org,
        orjan.eide@....com, robin.murphy@....com, jajones@...dia.com,
        hridya@...gle.com, sspatil@...gle.com, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 2/2] dma-buf: heaps: Map system heap pages as managed
 by linux vm

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@...nel.org>
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Reviewed-by: Minchan Kim <minchan@...nel.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).