linux-kernel - Re: [PATCH] MM: handle THP in swap_*page_fs()

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Message-ID: <059786bc-191b-8126-0c76-34dbf2e29159@huawei.com>
Date:   Mon, 9 May 2022 16:56:42 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Yang Shi <shy828301@...il.com>
CC:     NeilBrown <neilb@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Christoph Hellwig <hch@....de>, <linux-nfs@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH] MM: handle THP in swap_*page_fs() - count_vm_events()

On 2022/5/7 1:26, Yang Shi wrote:
> On Sun, May 1, 2022 at 10:32 PM Matthew Wilcox <willy@...radead.org> wrote:
>>
>> On Mon, May 02, 2022 at 03:28:49PM +1000, NeilBrown wrote:
>>> On Mon, 02 May 2022, Matthew Wilcox wrote:
>>>> On Mon, May 02, 2022 at 02:57:46PM +1000, NeilBrown wrote:
>>>>> @@ -390,9 +392,9 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>>>>>                   struct page *page = sio->bvec[p].bv_page;
>>>>>
>>>>>                   SetPageUptodate(page);
>>>>> +                 count_swpout_vm_event(page);
>>>>>                   unlock_page(page);
>>>>>           }
>>>>> -         count_vm_events(PSWPIN, sio->pages);
>>>>
>>>> Surely that should be count_swpIN_vm_event?
>>>>
>>> I'm not having a good day....
>>>
>>> Certainly shouldn't be swpout.  There isn't a count_swpin_vm_event().
>>>
>>> swap_readpage() only counts once for each page no matter how big it is.
>>> While swap_writepage() counts one for each PAGE_SIZE written.
>>>
>>> And we have THP_SWPOUT but not THP_SWPIN
>>
>> _If_ I understand the swap-in patch correctly (at least as invoked by
>> shmem), it won't attempt to swap in an entire THP.  Even if it swapped
>> out an order-9 page, it will bring in order-0 pages from swap, and then
>> rely on khugepaged to reassemble them.
> 
> Totally correct. The try_to_unmap() called by vmscan would split PMD
> to PTEs then install swap entries for each PTE but keep the huge page
> unsplit.
> 
> BTW, there were patches adding THP swapin support, but they were never merged.

Could you please tell me where the THP swapin patches are ? It would be really helpful
if you can kindly figure that out for me! :)

Thanks a lot!

> 
>>
>> Someone who actually understands the swap code should check that my
>> explanation here is correct.
>>
> .
>