Re: cma_alloc(), add sleep-and-retry for temporary page pinning

[Andrew Morton]

On Wed,  5 Aug 2020 19:56:21 -0700 Chris Goldsworthy <cgoldswo@xxxxxxxxxxxxxx> wrote:

> On mobile devices, failure to allocate from a CMA area constitutes a
> functional failure.  Sometimes during CMA allocations, we have observed
> that pages in a CMA area allocated through alloc_pages(), that we're trying
> to migrate away to make room for a CMA allocation, are temporarily pinned.
> This temporary pinning can occur when a process that owns the pinned page
> is being forked (the example is explained further in the commit text).
> This patch addresses this issue by adding a sleep-and-retry loop in
> cma_alloc() . There's another example we know of similar to the above that
> occurs during exit_mmap() (in zap_pte_range() specifically), but I need to
> determine if this is still relevant today.

Sounds fairly serious but boy, we're late for 5.9.

I can queue it for 5.10 with a cc:stable so that it gets backported
into earlier kernels a couple of months from now, if we think the
seriousness justifies backporting(?). 

Or I can do something else - thoughts?

And...  it really is a sad little patch, isn't it?  Instead of fixing
the problem, it reduces the problem's probability by 5x.  Can't we do
better than this?