Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

[Dan Williams]

On Sun, Jun 17, 2018 at 1:04 PM, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> On Sun, Jun 17, 2018 at 12:53:04PM -0700, Dan Williams wrote:
>> > diff --git a/mm/rmap.c b/mm/rmap.c
>> > index 6db729dc4c50..37576f0a4645 100644
>> > +++ b/mm/rmap.c
>> > @@ -1360,6 +1360,8 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>> >                                 flags & TTU_SPLIT_FREEZE, page);
>> >         }
>> >
>> > +       if (PageDmaPinned(page))
>> > +               return false;
>> >         /*
>> >          * We have to assume the worse case ie pmd for invalidation. Note that
>> >          * the page can not be free in this function as call of try_to_unmap()
>>
>> We have a similiar problem with DAX and the conclusion we came to is
>> that it is not acceptable for userspace to arbitrarily block kernel
>> actions. The conclusion there was: 'wait' if the DMA is transient, and
>> 'revoke' if the DMA is long lived, or otherwise 'block' long-lived DMA
>> if a revocation mechanism is not available.
>
> This might be the right answer for certain things, but it shouldn't be
> the immediate reaction to everthing. There are many user APIs that
> block kernel actions and hold kernel resources.
>
> IMHO, there should be an identifiable objection, eg is blocking going
> to create a DOS, dead-lock, insecurity, etc?

I believe kernel behavior regression is a primary concern as now
fallocate() and truncate() can randomly fail where they didn't before.