Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 7 May 2021 00:39:35 -0700 | Subject | Re: [PATCH 3/3] mm: gup: pack has_pinned in MMF_HAS_PINNED |
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On Thu, May 6, 2021 at 11:43 PM John Hubbard <jhubbard@nvidia.com> wrote: > > +static void set_mm_has_pinned_flag(unsigned long *mm_flags) > +{ > + /* > + * Avoid setting the bit unless necessary. This matters a lot with > + * large SMP machines. > + */ > + if (!test_bit(MMF_HAS_PINNED, mm_flags)) > + set_bit(MMF_HAS_PINNED, mm_flags); > +}
Yes, please do split it up like this.
But please make it explicitly inline, and move the comment to above the function.
And add the important key part to it: that the bit is never cleared.
That idempotent behavior of the "set_bit()" is what makes it safe to do this non-atomic test-and-set (yes, the "set_bit()" itself is atomic, but the sequence above is not).
Side note: we do have a few other places where this kind of thing happens, so it *might* make sense to even make this a generic pattern in case somebody can come up with a good descriptive name for that ("set_bit_if_not_set()" sounds descriptive, but the subtle non-atomicity should probably be part of it).
> + if (flags & FOLL_PIN) > + set_mm_has_pinned_flag(&mm->flags); > > ...which is now very readable, once again.
Yes, that does look much better.
Thanks,
Linus
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