Re: [PATCH 1/5] mm: reuse only-pte-mapped KSM page in do_wp_page()

[Suren Baghdasaryan]

On Thu, Apr 1, 2021 at 12:38 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Apr 01, 2021 at 11:17:37AM -0700, Suren Baghdasaryan wrote:
> > From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> >
> > Add an optimization for KSM pages almost in the same way that we have
> > for ordinary anonymous pages.  If there is a write fault in a page,
> > which is mapped to an only pte, and it is not related to swap cache; the
> > page may be reused without copying its content.
> >
> > [ Note that we do not consider PageSwapCache() pages at least for now,
> >   since we don't want to complicate __get_ksm_page(), which has nice
> >   optimization based on this (for the migration case). Currenly it is
> >   spinning on PageSwapCache() pages, waiting for when they have
> >   unfreezed counters (i.e., for the migration finish). But we don't want
> >   to make it also spinning on swap cache pages, which we try to reuse,
> >   since there is not a very high probability to reuse them. So, for now
> >   we do not consider PageSwapCache() pages at all. ]
> >
> > So in reuse_ksm_page() we check for 1) PageSwapCache() and 2)
> > page_stable_node(), to skip a page, which KSM is currently trying to
> > link to stable tree.  Then we do page_ref_freeze() to prohibit KSM to
> > merge one more page into the page, we are reusing.  After that, nobody
> > can refer to the reusing page: KSM skips !PageSwapCache() pages with
> > zero refcount; and the protection against of all other participants is
> > the same as for reused ordinary anon pages pte lock, page lock and
> > mmap_sem.
> >
> > [akpm@xxxxxxxxxxxxxxxxxxxx: replace BUG_ON()s with WARN_ON()s]
> > Link: http://lkml.kernel.org/r/154471491016.31352.1168978849911555609.stgit@localhost.localdomain
> > Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > Reviewed-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Cc: Christian Koenig <christian.koenig@xxxxxxx>
> > Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > ---
> >  include/linux/ksm.h |  7 +++++++
> >  mm/ksm.c            | 30 ++++++++++++++++++++++++++++--
> >  mm/memory.c         | 16 ++++++++++++++--
> >  3 files changed, 49 insertions(+), 4 deletions(-)
>
> You forgot to put the git commit id of the upstream commit in here
> somewhere so we can properly reference it and track it.
>
> When/if you resend this, please add it to all of the commits.

Will do. Thanks!

>
> thanks,
>
> greg k-h