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

Alex Shi
2020-07-17T05:09:43Z

>> @@ -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