Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

[Minchan Kim <minchan@kernel.org>]

On Wed, May 11, 2022 at 03:25:49PM -0700, John Hubbard wrote:
> On 5/11/22 2:46 PM, Minchan Kim wrote:
> > > I read that, but there was never any real justification there for needing
> > > to prevent a re-read of mt, just a preference: "I'd like to keep use the local
> > > variable mt's value in folloing conditions checks instead of refetching
> > > the value from get_pageblock_migratetype."
> > > 
> > > But I don't believe that there is any combination of values of mt that
> > > will cause a problem here.
> > > 
> > > I also think that once we pull in experts, they will tell us that the
> > > compiler is not going to re-run a non-trivial function to re-fetch a
> > > value, but I'm not one of those experts, so that's still arguable. But
> > > imagine what the kernel code would look like if every time we call
> > > a large function, we have to consider if it actually gets called some
> > > arbitrary number of times, due to (anti-) optimizations by the compiler.
> > > This seems like something that is not really happening.
> > 
> > Maybe, I might be paranoid since I have heard too subtle things
> > about how compiler could changes high level language code so wanted
> > be careful especially when we do lockless-stuff.
> > 
> > Who cares when we change the large(?) function to small(?) function
> > later on? I'd like to hear from experts to decide it.
> > 
> 
> Yes. But one thing that is still unanswered, that I think you can
> answer, is: even if the compiler *did* re-read the mt variable, what
> problems could that cause? I claim "no problems", because there is
> no combination of 0, _CMA, _ISOLATE, _CMA|ISOLATE that will cause
> problems here.

What scenario I am concerning with __READ_ONCE so compiler
inlining get_pageblock_migratetype two times are

        CPU 0                                                       CPU 1
                                                                alloc_contig_range
is_pinnable_page                                                start_isolate_page_range
                                                                  set_pageblock_migratetype(MIGRATE_ISOLATE)
   if (get_pageeblock_migratetype(page) == MIGRATE_CMA)    
       so it's false
                                                                undo:
                                                                  set_pageblock_migratetype(MIGRATE_CMA)
     
   if (get_pageeblock_migratetype(page) == MIGRATE_ISOLATE) 
       so it's false

In the end, CMA memory would be pinned by CPU 0 process
so CMA allocation keep failed until the process release the
refcount.

> 
> Any if that's true, then we can leave the experts alone, because
> the answer is there without knowing what happens exactly to mt.
> 
> thanks,
> 
> -- 
> John Hubbard
> NVIDIA
>