Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock

[Alex Shi]

在 2020/11/5 上午1:46, Johannes Weiner 写道:
> On Wed, Nov 04, 2020 at 07:27:21PM +0800, Alex Shi wrote:
>> 在 2020/11/3 上午4:20, Johannes Weiner 写道:
>>> On Mon, Nov 02, 2020 at 02:49:27PM +0000, Matthew Wilcox wrote:
>>>> On Mon, Nov 02, 2020 at 09:41:10AM -0500, Johannes Weiner wrote:
>>>>> On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote:
>>>>>> From: Hugh Dickins <hughd@xxxxxxxxxx>
>>>>>>
>>>>>> It is necessary for page_idle_get_page() to recheck PageLRU() after
>>>>>> get_page_unless_zero(), but holding lru_lock around that serves no
>>>>>> useful purpose, and adds to lru_lock contention: delete it.
>>>>>>
>>>>>> See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
>>>>>> discussion that led to lru_lock there; but __page_set_anon_rmap() now
>>>>>> uses WRITE_ONCE(),
>>>>>
>>>>> That doesn't seem to be the case in Linus's or Andrew's tree. Am I
>>>>> missing a dependent patch series?
>>>>>
>>>>>> and I see no other risk in page_idle_clear_pte_refs() using
>>>>>> rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly but
>>>>>> not entirely prevented by page_count() check in ksm.c's
>>>>>> write_protect_page(): that risk being shared with page_referenced()
>>>>>> and not helped by lru_lock).
>>>>>
>>>>> Isn't it possible, as per Minchan's description, for page->mapping to
>>>>> point to a struct anon_vma without PAGE_MAPPING_ANON set, and rmap
>>>>> thinking it's looking at a struct address_space?
>>>>
>>>> I don't think it can point to an anon_vma without the ANON bit set.
>>>> Minchan's concern in that email was that it might still be NULL.
>>>
>>> Hm, no, the thread is a lengthy discussion about whether the store
>>> could be split such that page->mapping is actually pointing to
>>> something invalid (anon_vma without the PageAnon bit).
>>>
>>> From his email:
>>>
>>>         CPU 0                                                                           CPU 1
>>>
>>> do_anonymous_page
>>>   __page_set_anon_rmap
>>>   /* out of order happened so SetPageLRU is done ahead */
>>>   SetPageLRU(page)
>>
>> This SetPageLRU done in __pagevec_lru_add_fn() which under the lru_lock
>> protection, so the original memory barrier or lock concern isn't
>> correct. that means, the SetPageLRU isn't possible to be here.
>> And then no warry on right side 'CPU 1' problem.
> 
> The SetPageLRU is done under lru_lock, but the store to page->mapping
> is not, so what ensures ordering between them? And what prevents the
> compiler from tearing the store to page->mapping?
> 

Right, I misunderstand the spin_lock in memory barrier. Thanks a lot
for point out this.
So, is this patch fine to fix the problem?