linux-kernel - Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page

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Date:   Thu, 1 Sep 2022 08:59:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        inuxppc-dev@...ts.ozlabs.org, linux-ia64@...r.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Michael Ellerman <mpe@...erman.id.au>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask

On 01.09.22 03:24, Baolin Wang wrote:
> 
> 
> On 9/1/2022 8:00 AM, Mike Kravetz wrote:
>> On 08/31/22 09:07, Baolin Wang wrote:
>>>
>>>
>>> On 8/31/2022 2:39 AM, Mike Kravetz wrote:
>>>> On 08/30/22 09:44, Mike Kravetz wrote:
>>>>> On 08/30/22 09:06, Baolin Wang wrote:
>>>>>> Hi Mike,
>>>>>>
>>>>>> On 8/30/2022 7:40 AM, Mike Kravetz wrote:
>>>>>>> During discussions of this series [1], it was suggested that hugetlb
>>>>>>> handling code in follow_page_mask could be simplified.  At the beginning
>>>>>>> of follow_page_mask, there currently is a call to follow_huge_addr which
>>>>>>> 'may' handle hugetlb pages.  ia64 is the only architecture which provides
>>>>>>> a follow_huge_addr routine that does not return error.  Instead, at each
>>>>>>> level of the page table a check is made for a hugetlb entry.  If a hugetlb
>>>>>>> entry is found, a call to a routine associated with that entry is made.
>>>>>>>
>>>>>>> Currently, there are two checks for hugetlb entries at each page table
>>>>>>> level.  The first check is of the form:
>>>>>>> 	if (p?d_huge())
>>>>>>> 		page = follow_huge_p?d();
>>>>>>> the second check is of the form:
>>>>>>> 	if (is_hugepd())
>>>>>>> 		page = follow_huge_pd().
>>>>>>>
>>>>>>> We can replace these checks, as well as the special handling routines
>>>>>>> such as follow_huge_p?d() and follow_huge_pd() with a single routine to
>>>>>>> handle hugetlb vmas.
>>>>>>>
>>>>>>> A new routine hugetlb_follow_page_mask is called for hugetlb vmas at the
>>>>>>> beginning of follow_page_mask.  hugetlb_follow_page_mask will use the
>>>>>>> existing routine huge_pte_offset to walk page tables looking for hugetlb
>>>>>>> entries.  huge_pte_offset can be overwritten by architectures, and already
>>>>>>> handles special cases such as hugepd entries.
>>>>>>
>>>>>> Could you also mention that this patch will fix the lock issue for
>>>>>> CONT-PTE/PMD hugetlb by changing to use huge_pte_lock()? which will help
>>>>>> people to understand the issue.
>>>>>
>>>>> Will update message in v2.  Thanks for taking a look!
>>>>>
>>>>
>>>> One additional thought, we 'may' need a separate patch to fix the locking
>>>> issues that can be easily backported.  Not sure this 'simplification' is
>>>> a good backport candidate.
>>>
>>> Yes, that was my thought before, but David did not like adding more
>>> make-legacy-cruft-happy code.
>>>
>>> So how about creating a series that contains 3 patches: picking up patch 1
>>> and patch 3 of my previous series [1], and your current patch? That means
>>> patch 1 and patch 2 in this series can fix the lock issue explicitly and be
>>> suitable to backport, meanwhile patch 3 (which is your current patch) will
>>> cleanup the legacy code.
>>>
>>
>> When I looked at patch 3, I was thinking the update follow_huge_pmd routine
>> would work for the PTE level with a few more modifications.  Perhaps, this is
>> too ugly but it is a smaller set of changes for backport.
>>
>> Of course, this would be followed up with the simplification patch which
>> removes all this code.
> 
> Yes, looks more simple. I can send you a formal patch with your 
> suggestion, which can be added into your cleanup series. Thanks.

As an alternative, we can have a stable-only version that does that.

-- 
Thanks,

David / dhildenb