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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Feb 4, 2021 at 7:55 AM Alex Deucher <alexdeucher@gmail.com> wrote:
>
> On Thu, Feb 4, 2021 at 3:16 AM Christian König <christian.koenig@amd.com> 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.

Thanks for the pointer!
Appreciate the time everyone took to explain the issues.
Thanks,
Suren.

>
> 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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