linux-kernel - Re: [PATCH v4 1/5] mm/rmap: support move to different root anon_vma in folio_move_anon

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Date:   Mon, 30 Oct 2023 16:15:13 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        lokeshgidra@...gle.com, david@...hat.com, hughd@...gle.com,
        mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
        willy@...radead.org, Liam.Howlett@...cle.com, jannh@...gle.com,
        zhangpeng362@...wei.com, bgeffon@...gle.com,
        kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 1/5] mm/rmap: support move to different root anon_vma
 in folio_move_anon_rmap()

On Fri, Oct 27, 2023 at 05:38:11PM -0700, Suren Baghdasaryan wrote:
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> For now, folio_move_anon_rmap() was only used to move a folio to a
> different anon_vma after fork(), whereby the root anon_vma stayed
> unchanged. For that, it was sufficient to hold the folio lock when
> calling folio_move_anon_rmap().
> 
> However, we want to make use of folio_move_anon_rmap() to move folios
> between VMAs that have a different root anon_vma. As folio_referenced()
> performs an RMAP walk without holding the folio lock but only holding the
> anon_vma in read mode, holding the folio lock is insufficient.
> 
> When moving to an anon_vma with a different root anon_vma, we'll have to
> hold both, the folio lock and the anon_vma lock in write mode.
> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to
> read-lock the anon_vma, we have to re-check if the mapping was changed
> in the meantime. If that was the case, we have to retry.
> 
> Note that folio_move_anon_rmap() must only be called if the anon page is
> exclusive to a process, and must not be called on KSM folios.
> 
> This is a preparation for UFFDIO_MOVE, which will hold the folio lock,
> the anon_vma lock in write mode, and the mmap_lock in read mode.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

Acked-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu