Re: [PATCH v16 15/22] mm/compaction: do page isolation first in compaction

[Alex Shi]

>> @@ -950,6 +951,21 @@ static bool too_many_isolated(pg_data_t *pgdat)
>>                 if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
>>                         goto isolate_fail;
>>
>> +               /*
>> +                * Be careful not to clear PageLRU until after we're
>> +                * sure the page is not being freed elsewhere -- the
>> +                * page release code relies on it.
>> +                */
>> +               if (unlikely(!get_page_unless_zero(page)))
>> +                       goto isolate_fail;
>> +
>> +               if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
>> +                       goto isolate_fail_put;
>> +
>> +               /* Try isolate the page */
>> +               if (!TestClearPageLRU(page))
>> +                       goto isolate_fail_put;
>> +
>>                 /* If we already hold the lock, we can skip some rechecking */
>>                 if (!locked) {
>>                         locked = compact_lock_irqsave(&pgdat->lru_lock,
> 
> Why not do the __isolate_lru_page_prepare before getting the page?
> That way you can avoid performing an extra atomic operation on non-LRU
> pages.
>

This change come from Hugh Dickins as mentioned from commit log:
>> trylock_page() is not safe to use at this time: its setting PG_locked
>> can race with the page being freed or allocated ("Bad page"), and can
>> also erase flags being set by one of those "sole owners" of a freshly
>> allocated page who use non-atomic __SetPageFlag().

Hi Hugh,

would you like to show more details of the bug?

...

>> +                        * sure the page is not being freed elsewhere -- the
>> +                        * page release code relies on it.
>> +                        */
>> +                       if (unlikely(!get_page_unless_zero(page)))
>> +                               goto busy;
>> +
>> +                       if (!TestClearPageLRU(page)) {
>> +                               /*
>> +                                * This page may in other isolation path,
>> +                                * but we still hold lru_lock.
>> +                                */
>> +                               put_page(page);
>> +                               goto busy;
>> +                       }
>> +
> 
> I wonder if it wouldn't make sense to combine these two atomic ops
> with tests and the put_page into a single inline function? Then it
> could be possible to just do one check and if succeeds you do the
> block of code below, otherwise you just fall-through into the -EBUSY
> case.
> 

Uh, since get_page changes page->_refcount, TestClearPageLRU changes page->flags,
So I don't know how to combine them, could you make it more clear with code?

Thanks
Alex