linux-kernel - Re: [PATCH 1/5] mm: reuse only-pte-mapped KSM page in do_wp

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Date:   Thu, 1 Apr 2021 21:38:15 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     stable@...r.kernel.org, jannh@...gle.com, ktkhai@...tuozzo.com,
        torvalds@...ux-foundation.org, shli@...com, namit@...are.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Yang Shi <yang.shi@...ux.alibaba.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christian Koenig <christian.koenig@....com>,
        Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>,
        Rik van Riel <riel@...riel.com>,
        Huang Ying <ying.huang@...el.com>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/5] mm: reuse only-pte-mapped KSM page in do_wp_page()

On Thu, Apr 01, 2021 at 11:17:37AM -0700, Suren Baghdasaryan wrote:
> From: Kirill Tkhai <ktkhai@...tuozzo.com>
> 
> 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@...ux-foundation.org: 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@...tuozzo.com>
> Reviewed-by: Yang Shi <yang.shi@...ux.alibaba.com>
> Cc: "Kirill A. Shutemov" <kirill@...temov.name>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Christian Koenig <christian.koenig@....com>
> Cc: Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Kirill Tkhai <ktkhai@...tuozzo.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> ---
>  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