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    SubjectRe: [PATCH 3/3] mm/vmscan: replace implicit RECLAIM_ZONE checks with explicit checks
    On Wed, 1 Jul 2020, Dave Hansen wrote:

    > On 7/1/20 1:04 PM, Ben Widawsky wrote:
    > >> +static inline bool node_reclaim_enabled(void)
    > >> +{
    > >> + /* Is any node_reclaim_mode bit set? */
    > >> + return node_reclaim_mode & (RECLAIM_ZONE|RECLAIM_WRITE|RECLAIM_UNMAP);
    > >> +}
    > >> +
    > >> extern void check_move_unevictable_pages(struct pagevec *pvec);
    > >>
    > >> extern int kswapd_run(int nid);
    > > If a user writes a bit that isn't a RECLAIM_* bit to vm.zone_reclaim_mode
    > > today, it acts as though RECLAIM_ZONE is enabled: we try to reclaim in
    > > zonelist order before falling back to the next zone in the page allocator.
    > > The sysctl doesn't enforce any max value :/ I dont know if there is any
    > > such user, but this would break them if there is.
    > >
    > > Should this simply be return !!node_reclaim_mode?
    >
    > You're right that there _could_ be a user-visible behavior change here.
    > But, if there were a change it would be for a bit which wasn't even
    > mentioned in the documentation. Somebody would have had to look at the
    > doc mentioning 1,2,4 and written an 8. If they did that, they're asking
    > for trouble because we could have defined the '8' bit to do nasty things
    > like auto-demote all your memory. :)
    >
    > I'll mention it in the changelog, but I still think we should check the
    > actual, known bits rather than check for 0.
    >
    > BTW, in the hardware, they almost invariably make unused bits "reserved"
    > and do mean things like #GP if someone tries to set them. This is a
    > case where the kernel probably should have done the same. It would have
    > saved us the trouble of asking these questions now. Maybe we should
    > even do that going forward.
    >

    Maybe enforce it in a sysctl handler so the user catches any errors, which
    would be better than silently accepting some policy that doesn't exist?

    RECLAIM_UNMAP and/or RECLAIM_WRITE should likely get -EINVAL if attempted
    to be set without RECLAIM_ZONE as well: they are no-ops without
    RECLAIM_ZONE. This would likely have caught something wrong with commit
    648b5cf368e0 ("mm/vmscan: remove unused RECLAIM_OFF/RECLAIM_ZONE") if it
    would have already been in place.

    I don't feel strongly about this, so feel free to ignore.

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