linux-kernel - Re: [PATCH 1/2] mm: protect free_pgtables with mmap_lock write lock in exit

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Date:   Tue, 23 Nov 2021 13:19:27 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, mhocko@...e.com,
        rientjes@...gle.com, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 1/2] mm: protect free_pgtables with mmap_lock write lock
 in exit_mmap

On Tue, Nov 16, 2021 at 01:57:14PM -0800, Suren Baghdasaryan wrote:
> @@ -3170,6 +3172,7 @@ void exit_mmap(struct mm_struct *mm)
>  	unmap_vmas(&tlb, vma, 0, -1);
>  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
>  	tlb_finish_mmu(&tlb);
> +	mmap_write_unlock(mm);
>  
>  	/*
>  	 * Walk the list again, actually closing and freeing it,

Is there a reason to unlock here instead of after the remove_vma loop?
We'll need the mmap sem held during that loop when VMAs are stored in
the maple tree.