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

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Message-ID: <0a8025eb-cf15-453a-7d7d-7c72d008889a@linux.alibaba.com>
Date:   Wed, 31 Aug 2022 09:07:28 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     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,
        David Hildenbrand <david@...hat.com>,
        "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 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.

[1] 
https://lore.kernel.org/all/cover.1661240170.git.baolin.wang@linux.alibaba.com/