Re: [PATCH rfc 0/2] mm: cma: make cma_release() non-blocking

[Roman Gushchin <guro@fb.com>]

On Thu, Oct 22, 2020 at 09:42:11AM -0700, Mike Kravetz wrote:
> On 10/21/20 7:33 PM, Roman Gushchin wrote:
> > On Wed, Oct 21, 2020 at 05:15:53PM -0700, Mike Kravetz wrote:
> >> On 10/16/20 3:52 PM, Roman Gushchin wrote:
> >>> This small patchset makes cma_release() non-blocking and simplifies
> >>> the code in hugetlbfs, where previously we had to temporarily drop
> >>> hugetlb_lock around the cma_release() call.
> >>>
> >>> It should help Zi Yan on his work on 1 GB THPs: splitting a gigantic
> >>> THP under a memory pressure requires a cma_release() call. If it's
> >>> a blocking function, it complicates the already complicated code.
> >>> Because there are at least two use cases like this (hugetlbfs is
> >>> another example), I believe it's just better to make cma_release()
> >>> non-blocking.
> >>>
> >>> It also makes it more consistent with other memory releasing functions
> >>> in the kernel: most of them are non-blocking.
> >>
> >> Thanks for looking into this Roman.
> > 
> > Hi Mike,
> > 
> >>
> >> I may be missing something, but why does cma_release have to be blocking
> >> today?  Certainly, it takes the bitmap in cma_clear_bitmap and could
> >> block.  However, I do not see why cma->lock has to be a mutex.  I may be
> >> missing something, but I do not see any code protected by the mutex doing
> >> anything that could sleep?
> >>
> >> Could we simply change that mutex to a spinlock?
> > 
> > I actually have suggested it few months ago, but the idea was
> > opposed by Joonsoo: https://lkml.org/lkml/2020/4/3/12 .
> > 
> > The time of a bitmap operation is definitely not an issue in my context,
> > but I can't speak for something like an embedded/rt case.
> > 
> 
> I wonder if it may be time to look into replacing the cma area bitmap
> with some other data structure?  Joonsoo was concerned about the time
> required to traverse the bitmap for an 8GB area.  With new support for
> allocating 1GB hugetlb pages from cma, I can imagine someone setting
> up a cma area that is hundreds of GB if not TB in size.  It is going
> take some time to traverse a bitmap describing such a huge area.

If the cma area is used exclusively for 1 GB allocations, the bitmap can
have only 1 bit per GB, so it shouldn't be a big problem.

Long-term I have some hopes to be able to allocate 1 GB pages without
a need to reserve a cma area: we can try to group pages based on their mobility
on a 1 GB scale, so that all non-movable pages will reside in few 1 GB blocks.
I'm looking into that direction, but don't have any results yet.

If this idea fails and we'll end up allocating a large cma area unconditionally
and shrink it on demand (I think Rik suggested something like this),
replacing the bitmap with something else sounds like a good idea to me.

As now, I want to unblock Zi Yan on his work on 1 GB THPs, so maybe
we can go with introducing cma_release_nowait(), as I suggested in
the other e-mail in this thread? Do you have any objections?

Thanks!