Re: [PATCH v3 01/16] mm: use put_page to free page instead of putback_lru_page

[Vlastimil Babka <vbabka@suse.cz>]

On 04/04/2016 06:45 AM, Naoya Horiguchi wrote:
> On Mon, Apr 04, 2016 at 10:39:17AM +0900, Minchan Kim wrote:
>> Thanks for catching it, Vlastimil.
>> It was my mistake. But in this chance, I looked over hwpoison code and
>> I saw other places which increases num_poisoned_pages are successful
>> migration, already freed page and successful invalidated page.
>> IOW, they are already successful isolated page so I guess it should
>> increase the count when only successful migration is done?
> 
> Yes, that's right. When exiting with migration's failure, we shouldn't call
> test_set_page_hwpoison or num_poisoned_pages_inc, so current code checking
> (rc != -EAGAIN) is simply incorrect. Your change fixes the bug in memory
> error handling. Great!

Ah, I see, soft onlining works differently than I thought.

>> And when I read memory_failure, it bails out without killing if it
>> encounters HWPoisoned page so I think it's not for catching and
>> kill the poor proces.
>>
>>>
>>> Also (but not your fault) the put_page() preceding
>>> test_set_page_hwpoison(page)) IMHO deserves a comment saying which
>>> pin we are releasing and which one we still have (hopefully? if I
>>> read description of da1b13ccfbebe right) otherwise it looks like
>>> doing something with a page that we just potentially freed.
>>
>> Yes, while I read the code, I had same question. I think the releasing
>> refcount is for get_any_page.
> 
> As the other callers of page migration do, soft_offline_page expects the
> migration source page to be freed at this put_page() (no pin remains.)
> The refcount released here is from isolate_lru_page() in __soft_offline_page().
> (the pin by get_any_page is released by put_hwpoison_page just after it.)
> 
> .. yes, doing something just after freeing page looks weird, but that's
> how PageHWPoison flag works. IOW, many other page flags are maintained
> only during one "allocate-free" life span, but PageHWPoison still does
> its job beyond it.

But what prevents the page from being allocated again between put_page()
and test_set_page_hwpoison()? In that case we would be marking page
poisoned while still in use, which is the same as marking it while still
in use after a failed migration?

(Also, which part prevents pages with PageHWPoison to be allocated
again, anyway? I can't find it and test_set_page_hwpoison() doesn't
remove from buddy freelists).

Thanks.

> As for commenting, this put_page() is called in any MIGRATEPAGE_SUCCESS
> case (regardless of callers), so what we can say here is "we free the
> source page here, bypassing LRU list" or something?
> 
> Thanks,
> Naoya Horiguchi
>