Messages in this thread | | | Date | Wed, 11 May 2022 16:57:04 -0700 | Subject | Re: [PATCH v4] mm: fix is_pinnable_page against on cma page | From | John Hubbard <> |
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On 5/11/22 16:45, Paul E. McKenney wrote: >> >> Well no, because the "&" operation is a single operation on the CPU, and >> isn't going to get split up like that. > > Chiming in a bit late...
Much appreciated!
> > The usual way that this sort of thing causes trouble is if there is a > single store instruction that changes the value from MIGRATE_ISOLATE > to MIGRATE_CMA, and if the compiler decides to fetch twice, AND twice,
Doing an AND twice for "x & constant" this definitely blows my mind. Is nothing sacred? :)
> and then combine the results. This could give a zero outcome where the > underlying variable never had the value zero. > > Is this sort of thing low probability? > > Definitely. > > Isn't this sort of thing prohibited? > > Definitely not. > > So what you have will likely work for at least a while longer, but it > is not guaranteed and it forces you to think a lot harder about what > the current implementations of the compiler can and cannot do to you. > > The following LWN article goes through some of the possible optimizations > (vandalisms?) in this area: https://lwn.net/Articles/793253/ >
hmm, I don't think we hit any of those cases, do we? Because here, the "write" side is via a non-inline function that I just don't believe the compiler is allowed to call twice. Or is it?
Minchan's earlier summary:
CPU 0 CPU1
set_pageblock_migratetype(MIGRATE_ISOLATE)
if (get_pageblock_migrate(page) & MIGRATE_CMA)
set_pageblock_migratetype(MIGRATE_CMA)
if (get_pageblock_migrate(page) & MIGRATE_ISOLATE)
...where set_pageblock_migratetype() is not inline.
thanks, -- John Hubbard NVIDIA
> In the end, it is your code, so you get to decide how much you would > like to keep track of what compilers get up to over time. ;-) > > Thanx, Paul
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