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From: David Hildenbrand <david@redhat.com>
To: Alistair Popple <apopple@nvidia.com>, Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Jerome Glisse <jglisse@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Miaohe Lin <linmiaohe@huawei.com>, Yang Shi <shy828301@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 4/5] mm: Add zap_skip_check_mapping() helper
Date: Fri, 3 Sep 2021 09:07:23 +0200	[thread overview]
Message-ID: <072aa3aa-e4f8-96f0-50b5-92707404926d@redhat.com> (raw)
In-Reply-To: <2306207.EELmk4mpEQ@nvdebian>

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@redhat.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


  reply	other threads:[~2021-09-03  7:07 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02 20:17 [PATCH v2 0/5] mm: A few cleanup patches around zap, shmem and uffd Peter Xu
2021-09-02 20:17 ` [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in mfill_atomic_install_pte Peter Xu
2021-09-03  7:42   ` David Hildenbrand
2021-09-03 20:00     ` Peter Xu
2021-09-03 20:02       ` David Hildenbrand
2021-09-02 20:17 ` [PATCH v2 2/5] mm: Clear vmf->pte after pte_unmap_same() returns Peter Xu
2021-09-08  1:12   ` Liam Howlett
2021-09-02 20:17 ` [PATCH v2 3/5] mm: Drop first_index/last_index in zap_details Peter Xu
2021-09-02 20:18 ` [PATCH v2 4/5] mm: Add zap_skip_check_mapping() helper Peter Xu
2021-09-03  0:58   ` Alistair Popple
2021-09-03  1:39     ` Peter Xu
2021-09-03  1:50       ` Alistair Popple
2021-09-03  7:07         ` David Hildenbrand [this message]
2021-09-02 20:18 ` [PATCH v2 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags Peter Xu
2021-09-03  7:25   ` David Hildenbrand
2021-09-03  7:31     ` David Hildenbrand
2021-09-08  0:43     ` Peter Xu
2021-09-08  0:35 ` [PATCH v2 0/5] mm: A few cleanup patches around zap, shmem and uffd Peter Xu

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