Messages in this thread | | | Subject | Re: [PATCH] mm/page_isolation: don't putback unisolated page | From | David Hildenbrand <> | Date | Tue, 7 Sep 2021 11:56:20 +0200 |
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On 07.09.21 10:08, Vlastimil Babka wrote: > On 9/6/21 14:49, David Hildenbrand wrote: >> On 06.09.21 14:45, Miaohe Lin wrote: >>> On 2021/9/6 20:11, David Hildenbrand wrote: >>>> On 06.09.21 14:02, David Hildenbrand wrote: >>>>> On 04.09.21 11:18, Miaohe Lin wrote: >>>>> >>>>> Thanks! >>>>> >>>>> Reviewed-by: David Hildenbrand <david@redhat.com> >>>>> >>>> >>>> To make the confusion perfect (sorry) :D I tripple-checked: >>>> >>>> In unset_migratetype_isolate() we check that is_migrate_isolate_page(page) holds, otherwise we return. >>>> >>>> We call __isolate_free_page() only for such pages. >>>> >>>> __isolate_free_page() won't perform watermark checks on is_migrate_isolate(). >>>> >>>> Consequently, __isolate_free_page() should never fail when called from unset_migratetype_isolate() >>>> >>>> If that's correct then we could instead maybe add a VM_BUG_ON() and a comment why this can't fail. >>>> >>>> >>>> Makes sense or am I missing something? >>> >>> I think you're right. __isolate_free_page() should never fail when called from unset_migratetype_isolate() >>> as explained by you. But it might be too fragile to reply on the failure conditions of __isolate_free_page(). >>> If that changes, VM_BUG_ON() here might trigger unexpectedly. Or am I just over-worried as failure conditions >>> of __isolate_free_page() can hardly change? >> >> Maybe >> >> isolated_page = !!__isolate_free_page(page, order); >> /* >> * Isolating a free page in an isolated pageblock is expected to always >> * work as watermarks don't apply here. >> */ >> VM_BUG_ON(isolated_page); >> >> >> VM_BUG_ON() allows us to detect any issues when testing. Combined with >> the comment it tells everybody messing with __isolate_free_page() what >> we expect in this function. >> >> In production system, we would handle it gracefully. > > If this can be handled gracefully, then I'd rather go with VM_WARN_ON. > Maybe even WARN_ON_ONCE? >
I think either VM_BUG_ON() or VM_WARN_ON() -- compiling the runtime checks out -- should be good enough.
I'd just go with VM_BUG_ON(), because anybody messing with __isolate_free_page() should clearly spot that we expect the current handling. But no strong opinion.
-- Thanks,
David / dhildenb
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