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
