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    SubjectRe: [page-reclaim] Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks
    On Fri, Apr 15, 2022 at 4:04 PM Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    >
    > On Fri, Apr 15, 2022 at 3:58 PM Yu Zhao <yuzhao@google.com> wrote:
    > >
    > > BUG_ONs are harmful but problems that trigger them would be
    > > presummingly less penetrating to the user base; on the other hand,
    > > from my experience working with some testers (ordinary users), they
    > > ignore WARN_ON_ONCEs until the kernel crashes.
    >
    > I don't understand your argument.
    >
    > First you say that VM_BUG_ON() is only for VM developers.
    >
    > Then you say "some testers (ordinary users) ignore WARN_ON_ONCEs until
    > the kernel crashes".
    >
    > So which is it?
    >
    > VM developers, or ordinary users?
    >
    > Honestly, if a VM developer is ignoring a WARN_ON_ONCE() from the VM
    > subsystem, I don't even know what to say.
    >
    > And for ordinary users, a WARN_ON_ONCE() is about a million times
    > better, becasue:
    >
    > - the machine will hopefully continue working, so they can report the warning
    >
    > - even when they don't notice them, distros tend to have automated
    > reporting infrastructure
    >
    > That's why I absolutely *DETEST* those stupid BUG_ON() cases - they
    > will often kill the machine with nasty locks held, resulting in a
    > completely undebuggable thing that never gets reported.
    >
    > Yes, you can be careful and only put BUG_ON() in places where recovery
    > is possible. But even then, they have no actual _advantages_ over just
    > a WARN_ON_ONCE.

    Generally agreed, and not to belabor this relatively small issue, but in some
    environments like cloud or managed client deployments, a crash can actually
    be preferable so we can get a dump, reboot the machine, and get things going
    again for the application or user, then debug offline. So having the
    flexibility to
    do that in those situations is helpful. And there, a full crash dump is better
    than just a log report with the WARN info, since debugging may be easier with
    all the kernel memory.

    Jesse

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