Re: [PATCH 10/12] mm: page migration use migration entry for swapcache too

Cyrill Gorcunov
2015-10-22T22:36:03Z

On Sun, Oct 18, 2015 at 10:05:28PM -0700, Hugh Dickins wrote:
> Hitherto page migration has avoided using a migration entry for a
> swapcache page mapped into userspace, apparently for historical reasons.
> So any page blessed with swapcache would entail a minor fault when it's
> next touched, which page migration otherwise tries to avoid. Swapcache
> in an mlocked area is rare, so won't often matter, but still better fixed.
>
> Just rearrange the block in try_to_unmap_one(), to handle TTU_MIGRATION
> before checking PageAnon, that's all (apart from some reindenting).
>
> Well, no, that's not quite all: doesn't this by the way fix a soft_dirty
> bug, that page migration of a file page was forgetting to transfer the
> soft_dirty bit? Probably not a serious bug: if I understand correctly,
> soft_dirty afficionados usually have to handle file pages separately
> anyway; but we publish the bit in /proc/<pid>/pagemap on file mappings
> as well as anonymous, so page migration ought not to perturb it.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Sorry for delay in response. Indeed this should fix the nit, thanks!
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>