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

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Message-ID: <Yw5ZTEqxZCAgEbK9@monkey>
Date:   Tue, 30 Aug 2022 11:39:08 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.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 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.
-- 
Mike Kravetz