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

From: Greg KH
Date: Thu Apr 01 2021 - 15:38:24 EST


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.

thanks,

greg k-h