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
