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

Miaohe Lin
2022-04-14T02:40:28Z

On 2022/4/13 20:31, David Hildenbrand wrote:
> 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.

That's a pity. We really need a asynchronous writeback to 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.
>

ZRAM_WRITEBACK might need to be configured to enable asynchronous IO:

+
+config ZRAM_WRITEBACK
+ bool "Write back incompressible page to backing device"
+ depends on ZRAM
+ default n
+ help
+ With incompressible page, there is no memory saving to keep it
+ in memory. Instead, write it out to backing device.
+ For this feature, admin should set up backing device via
+ /sys/block/zramX/backing_dev.
+
+ See zram.txt for more infomration.

It seems there is only asynchronous IO for swapin ops. I browsed the source code
and I can only found read_from_bdev_async. But I'm not familiar with the zram code.
Minchan might could kindly help us solving this question.

Thanks!