Re: [PATCH] mm/page_isolation: don't putback unisolated page

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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@xxxxxxxxxx>


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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