Re: [PATCH v2 1/8] mm/swap: remember PG_anon_exclusive via a swp pte bit

[David Hildenbrand <david@redhat.com>]

On 13.04.22 11:38, Miaohe Lin wrote:
> On 2022/4/13 17:30, David Hildenbrand wrote:
>> On 13.04.22 10:58, Miaohe Lin wrote:
>>> On 2022/3/30 0:43, David Hildenbrand wrote:
>>>> Currently, we clear PG_anon_exclusive in try_to_unmap() and forget about
>>> ...
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 14618f446139..9060cc7f2123 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -792,6 +792,11 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>>>>  						&src_mm->mmlist);
>>>>  			spin_unlock(&mmlist_lock);
>>>>  		}
>>>> +		/* Mark the swap entry as shared. */
>>>> +		if (pte_swp_exclusive(*src_pte)) {
>>>> +			pte = pte_swp_clear_exclusive(*src_pte);
>>>> +			set_pte_at(src_mm, addr, src_pte, pte);
>>>> +		}
>>>>  		rss[MM_SWAPENTS]++;
>>>>  	} else if (is_migration_entry(entry)) {
>>>>  		page = pfn_swap_entry_to_page(entry);
>>>> @@ -3559,6 +3564,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>>  	struct page *page = NULL, *swapcache;
>>>>  	struct swap_info_struct *si = NULL;
>>>>  	rmap_t rmap_flags = RMAP_NONE;
>>>> +	bool exclusive = false;
>>>>  	swp_entry_t entry;
>>>>  	pte_t pte;
>>>>  	int locked;
>>>> @@ -3724,6 +3730,46 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>>  	BUG_ON(!PageAnon(page) && PageMappedToDisk(page));
>>>>  	BUG_ON(PageAnon(page) && PageAnonExclusive(page));
>>>>  
>>>> +	/*
>>>> +	 * Check under PT lock (to protect against concurrent fork() sharing
>>>> +	 * the swap entry concurrently) for certainly exclusive pages.
>>>> +	 */
>>>> +	if (!PageKsm(page)) {
>>>> +		/*
>>>> +		 * Note that pte_swp_exclusive() == false for architectures
>>>> +		 * without __HAVE_ARCH_PTE_SWP_EXCLUSIVE.
>>>> +		 */
>>>> +		exclusive = pte_swp_exclusive(vmf->orig_pte);
>>>> +		if (page != swapcache) {
>>>> +			/*
>>>> +			 * We have a fresh page that is not exposed to the
>>>> +			 * swapcache -> certainly exclusive.
>>>> +			 */
>>>> +			exclusive = true;
>>>> +		} else if (exclusive && PageWriteback(page) &&
>>>> +			   !(swp_swap_info(entry)->flags & SWP_STABLE_WRITES)) {
>>>
>>> Really sorry for late respond and a newbie question. IIUC, if SWP_STABLE_WRITES is set,
>>> it means concurrent page modifications while under writeback is not supported. For these
>>> problematic swap backends, exclusive marker is dropped. So the above if statement is to
>>> filter out these problematic swap backends which have SWP_STABLE_WRITES set. If so, the
>>> above check should be && (swp_swap_info(entry)->flags & SWP_STABLE_WRITES)), i.e. no "!".
>>> Or am I miss something?
>>
>> Oh, thanks for your careful eyes!
>>
>> Indeed, SWP_STABLE_WRITES indicates that the backend *requires* stable
>> writes, meaning, we must not modify the page while writeback is active.
>>
>> So if and only if that is set, we must drop the exclusive marker.
>>
>> This essentially corresponds to previous reuse_swap_page() logic:
>>
>> bool reuse_swap_page(struct page *page)
>> {
>> ...
>> 	if (!PageWriteback(page)) {
>> 		...
>> 	} else {
>> 		...
>> 		if (p->flags & SWP_STABLE_WRITES) {
>> 			spin_unlock(&p->lock);
>> 			return false;
>> 		}
>> ...
>> }
>>
>> Fortunately, this only affects such backends. For backends without
>> SWP_STABLE_WRITES, the current code is simply sub-optimal.
>>
>>
>> So yes, this has to be
>>
>> } else if (exclusive && PageWriteback(page) &&
>> 	   (swp_swap_info(entry)->flags & SWP_STABLE_WRITES)) {
>>
> 
> I am glad that my question helps. :)
> 
>>
>> Let me try finding a way to test this, the tests I was running so far
>> were apparently not using a backend with SWP_STABLE_WRITES.
>>
> 
> That will be really helpful. Many thanks for your hard work!
> 

FWIW, I tried with zram, which sets SWP_STABLE_WRITES ... but, it seems
to always do a synchronous writeback, so it cannot really trigger this
code path.

commit f05714293a591038304ddae7cb0dd747bb3786cc
Author: Minchan Kim <minchan@kernel.org>
Date:   Tue Jan 10 16:58:15 2017 -0800

    mm: support anonymous stable page


mentions "During developemnt for zram-swap asynchronous writeback,";
maybe that can be activated somehow? Putting Minchan on CC.

-- 
Thanks,

David / dhildenb