linux-kernel - Re: [PATCH 4/7] mm/khugepaged: minor cleanup for collapse

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Date:   Thu, 16 Jun 2022 14:10:48 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Yang Shi <shy828301@...il.com>, Zach O'Keefe <zokeefe@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Howells <dhowells@...hat.com>, NeilBrown <neilb@...e.de>,
        Alistair Popple <apopple@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Peter Xu <peterx@...hat.com>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] mm/khugepaged: minor cleanup for collapse_file

On 2022/6/16 2:18, Yang Shi wrote:
> On Wed, Jun 15, 2022 at 8:55 AM Zach O'Keefe <zokeefe@...gle.com> wrote:
>>
>> On 11 Jun 16:47, Miaohe Lin wrote:
>>> nr_none is always 0 for non-shmem case because the page can be read from
>>> the backend store. So when nr_none ! = 0, it must be in is_shmem case.
>>> Also only adjust the nrpages and uncharge shmem when nr_none != 0 to save
>>> cpu cycles.
>>>
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>  mm/khugepaged.c | 9 ++++-----
>>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 1b5dd3820eac..8e6fad7c7bd9 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -1885,8 +1885,7 @@ static void collapse_file(struct mm_struct *mm,
>>>
>>>       if (nr_none) {
>>>               __mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
>>> -             if (is_shmem)
>>> -                     __mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
>>> +             __mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
>>>       }
>>
>>
>> Might be worth a small comment here - even though folks can see in above code
>> that this is only incremented in shmem path, might be nice to say why it's
>> always 0 for non-shmem (or conversely, why it's only possible to be non 0 on
>> shmem).
> 
> Agreed, better to have some comments in the code.

Will try to add comments in next version. Thanks both!

> 
>>
>>>
>>>       /* Join all the small entries into a single multi-index entry */
>>> @@ -1950,10 +1949,10 @@ static void collapse_file(struct mm_struct *mm,
>>>
>>>               /* Something went wrong: roll back page cache changes */
>>>               xas_lock_irq(&xas);
>>> -             mapping->nrpages -= nr_none;
>>> -
>>> -             if (is_shmem)
>>> +             if (nr_none) {
>>> +                     mapping->nrpages -= nr_none;
>>>                       shmem_uncharge(mapping->host, nr_none);
>>> +             }
>>>
>>>               xas_set(&xas, start);
>>>               xas_for_each(&xas, page, end - 1) {
>>> --
>>> 2.23.0
>>>
>>>
>>
>> Otherwise,
>>
>> Reviewed-by: Zach O'Keefe <zokeefe@...gle.com>
>>
> .
>