linux-kernel - Re: [PATCH v2 4/4] mm:swap: skip swapcache for swapin of synchronous device

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Date:   Mon, 9 Oct 2017 09:26:18 +0800
From:   huang ying <huang.ying.caritas@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@....com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH v2 4/4] mm:swap: skip swapcache for swapin of synchronous device

On Fri, Sep 29, 2017 at 4:51 PM, huang ying
<huang.ying.caritas@...il.com> wrote:
> On Wed, Sep 20, 2017 at 1:43 PM, Minchan Kim <minchan@...nel.org> wrote:

[snip]

>> diff --git a/mm/memory.c b/mm/memory.c
>> index ec4e15494901..163ab2062385 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2842,7 +2842,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
>>  int do_swap_page(struct vm_fault *vmf)
>>  {
>>         struct vm_area_struct *vma = vmf->vma;
>> -       struct page *page = NULL, *swapcache;
>> +       struct page *page = NULL, *swapcache = NULL;
>>         struct mem_cgroup *memcg;
>>         struct vma_swap_readahead swap_ra;
>>         swp_entry_t entry;
>> @@ -2881,17 +2881,35 @@ int do_swap_page(struct vm_fault *vmf)
>>                 }
>>                 goto out;
>>         }
>> +
>> +
>>         delayacct_set_flag(DELAYACCT_PF_SWAPIN);
>>         if (!page)
>>                 page = lookup_swap_cache(entry, vma_readahead ? vma : NULL,
>>                                          vmf->address);
>>         if (!page) {
>> -               if (vma_readahead)
>> -                       page = do_swap_page_readahead(entry,
>> -                               GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
>> -               else
>> -                       page = swapin_readahead(entry,
>> -                               GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> +               struct swap_info_struct *si = swp_swap_info(entry);
>> +
>> +               if (!(si->flags & SWP_SYNCHRONOUS_IO)) {
>> +                       if (vma_readahead)
>> +                               page = do_swap_page_readahead(entry,
>> +                                       GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
>> +                       else
>> +                               page = swapin_readahead(entry,
>> +                                       GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> +                       swapcache = page;
>> +               } else {
>> +                       /* skip swapcache */
>> +                       page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> +                       if (page) {
>> +                               __SetPageLocked(page);
>> +                               __SetPageSwapBacked(page);
>> +                               set_page_private(page, entry.val);
>> +                               lru_cache_add_anon(page);
>> +                               swap_readpage(page, true);
>> +                       }
>> +               }
>
> I have a question for this.  If a page is mapped in multiple processes
> (for example, because of fork).  With swap cache, after swapping out
> and swapping in, the page will be still shared by these processes.
> But with your changes, it appears that there will be multiple pages
> with same contents mapped in multiple processes, even if the page
> isn't written in these processes.  So this may waste some memory in
> some situation?  And copying from device is even faster than looking
> up swap cache in your system?

Hi, Minchan,

Could you help me on this?

Best Regards,
Huang, Ying

[snip]