linux-kernel - Re: [PATCH 1/4] mm: pagewalk: assert write mmap lock only for walking the user page tables

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Message-ID: <20231204213943.GB4129@monkey>
Date:   Mon, 4 Dec 2023 13:39:43 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Steven Price <steven.price@....com>, Jann Horn <jannh@...gle.com>,
        muchun.song@...ux.dev, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] mm: pagewalk: assert write mmap lock only for
 walking the user page tables

On 11/27/23 16:46, Muchun Song wrote:
> The 8782fb61cc848 ("mm: pagewalk: Fix race between unmap and page walker")
> introduces an assertion to walk_page_range_novma() to make all the users
> of page table walker is safe. However, the race only exists for walking the
> user page tables. And it is ridiculous to hold a particular user mmap write
> lock against the changes of the kernel page tables. So only assert at least
> mmap read lock when walking the kernel page tables. And some users matching
> this case could downgrade to a mmap read lock to relief the contention of
> mmap lock of init_mm, it will be nicer in hugetlb (only holding mmap read
> lock) in the next patch.

At first, I did not understand your motivation for this patch.  But, it
makes sense as your next patch will replace hugetlb vmemmap specific walk
routines with walk_page_range_novma.

Directly Cc'ing Steven and Jann in case they have comments.  

> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/pagewalk.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)

Looks fine to me,
Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz