Re: [PATCH v4 1/5] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()

[Peter Xu]

On Fri, Oct 27, 2023 at 05:38:11PM -0700, Suren Baghdasaryan wrote:
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Acked-by: Peter Xu <peterx@xxxxxxxxxx>

-- 
Peter Xu