Re: [Linaro-mm-sig] [PATCH 1/2] mm: replace BUG_ON in vm_insert_page with a return of an error

[Alex Deucher]

On Thu, Feb 4, 2021 at 3:16 AM Christian König <christian.koenig@xxxxxxx> wrote:
>
> Am 03.02.21 um 22:41 schrieb Suren Baghdasaryan:
> > [SNIP]
> >>> How many semi-unrelated buffer accounting schemes does google come up with?
> >>>
> >>> We're at three with this one.
> >>>
> >>> And also we _cannot_ required that all dma-bufs are backed by struct
> >>> page, so requiring struct page to make this work is a no-go.
> >>>
> >>> Second, we do not want to all get_user_pages and friends to work on
> >>> dma-buf, it causes all kinds of pain. Yes on SoC where dma-buf are
> >>> exclusively in system memory you can maybe get away with this, but
> >>> dma-buf is supposed to work in more places than just Android SoCs.
> >> I just realized that vm_inser_page doesn't even work for CMA, it would
> >> upset get_user_pages pretty badly - you're trying to pin a page in
> >> ZONE_MOVEABLE but you can't move it because it's rather special.
> >> VM_SPECIAL is exactly meant to catch this stuff.
> > Thanks for the input, Daniel! Let me think about the cases you pointed out.
> >
> > IMHO, the issue with PSS is the difficulty of calculating this metric
> > without struct page usage. I don't think that problem becomes easier
> > if we use cgroups or any other API. I wanted to enable existing PSS
> > calculation mechanisms for the dmabufs known to be backed by struct
> > pages (since we know how the heap allocated that memory), but sounds
> > like this would lead to problems that I did not consider.
>
> Yeah, using struct page indeed won't work. We discussed that multiple
> times now and Daniel even has a patch to mangle the struct page pointers
> inside the sg_table object to prevent abuse in that direction.
>
> On the other hand I totally agree that we need to do something on this
> side which goes beyong what cgroups provide.
>
> A few years ago I came up with patches to improve the OOM killer to
> include resources bound to the processes through file descriptors. I
> unfortunately can't find them of hand any more and I'm currently to busy
> to dig them up.

https://lists.freedesktop.org/archives/dri-devel/2015-September/089778.html
I think there was a more recent discussion, but I can't seem to find it.

Alex

>
> In general I think we need to make it possible that both the in kernel
> OOM killer as well as userspace processes and handlers have access to
> that kind of data.
>
> The fdinfo approach as suggested in the other thread sounds like the
> easiest solution to me.
>
> Regards,
> Christian.
>
> > Thanks,
> > Suren.
> >
> >
>
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