linux-kernel - Re: [PATCH v2 4/5] mm: Add zap_skip_check

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Date:   Fri, 3 Sep 2021 09:07:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>, Peter Xu <peterx@...hat.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 4/5] mm: Add zap_skip_check_mapping() helper

On 03.09.21 03:50, Alistair Popple wrote:
> On Friday, 3 September 2021 11:39:32 AM AEST Peter Xu wrote:
>> On Fri, Sep 03, 2021 at 10:58:53AM +1000, Alistair Popple wrote:
>>> On Friday, 3 September 2021 6:18:19 AM AEST Peter Xu wrote:
>>>> Use the helper for the checks.  Rename "check_mapping" into "zap_mapping"
>>>> because "check_mapping" looks like a bool but in fact it stores the mapping
>>>> itself.  When it's set, we check the mapping (it must be non-NULL).  When it's
>>>> cleared we skip the check, which works like the old way.
>>>>
>>>> Move the duplicated comments to the helper too.
>>>>
>>>> Signed-off-by: Peter Xu <peterx@...hat.com>
>>>> ---
>>>>   include/linux/mm.h | 15 ++++++++++++++-
>>>>   mm/memory.c        | 29 ++++++-----------------------
>>>>   2 files changed, 20 insertions(+), 24 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 69259229f090..81e402a5fbc9 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -1720,10 +1720,23 @@ extern void user_shm_unlock(size_t, struct ucounts *);
>>>>    * Parameter block passed down to zap_pte_range in exceptional cases.
>>>>    */
>>>>   struct zap_details {
>>>> -	struct address_space *check_mapping;	/* Check page->mapping if set */
>>>> +	struct address_space *zap_mapping;	/* Check page->mapping if set */
>>>>   	struct page *single_page;		/* Locked page to be unmapped */
>>>>   };
>>>>   
>>>> +/*
>>>> + * We set details->zap_mappings when we want to unmap shared but keep private
>>>> + * pages. Return true if skip zapping this page, false otherwise.
>>>> + */
>>>> +static inline bool
>>>> +zap_skip_check_mapping(struct zap_details *details, struct page *page)
>>>> +{
>>>> +	if (!details || !page)
>>>> +		return false;
>>>> +
>>>> +	return details->zap_mapping != page_rmapping(page);
>>>
>>> Shouldn't this check still be
>>> details->zap_mapping && details->zap_mapping != page_rmapping(page)?
>>>
>>> Previously we wouldn't skip zapping pages if even_cows == true (ie.
>>> details->check_mapping == NULL). With this change the check when
>>> even_cows == true becomes NULL != page_rmapping(page). Doesn't this mean we
>>> will now skip zapping any pages with a mapping when even_cows == true?
>>
>> Yes I think so.  Thanks for pointing that out, Alistair, I'll fix in v3.
>>
>> But frankly I really think we should simply have that flag I used to introduce.
>> It'll make everything much clearer.
> 
> Yeah, I think a flag would also be fine.

I still don't see the need for a flag quite frankly. Just factor out the 
checks we already have ... no change in behavior.


-- 
Thanks,

David / dhildenb