Re: [PATCH 0/4] mm: Simplfy cow handling

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 22.08.2020 02:49, Peter Xu wrote:
> This is a small series that I picked up from Linus's suggestion [0] to simplify
> cow handling (and also more strict) by checking against page refcounts rather
> than mapcounts.
> 
> I'm CCing the author and reviewer of commit 52d1e606ee73 on ksm ("mm: reuse
> only-pte-mapped KSM page in do_wp_page()", 2019-03-05).  Please shoot if
> there's any reason to keep the logic, or it'll be removed in this series.  For
> more information, please refer to [3,4].

I'm not sure I understand the reasons to remove that. But the reason to add was
to avoid excess page copying, when it is not needed in real.
 
> The new mm counter in the last patch can be seen as RFC, depending on whether
> anyone dislikes it... I used it majorly for observing the page reuses, so it is
> kind of optional.
> 
> Two tests I did:
> 
>   - Run a busy loop dirty program [1] that uses 6G of memory, restrict to 1G
>     RAM + 5G swap (cgroup).  A few hours later, all things still look good.
>     Make sure to observe (still massive) correct page reuses using the new
>     counter using the last patch, probably when swapping in.
> 
>   - Run umapsort [2] to make sure uffd-wp will work again after applying this
>     series upon master 5.9-rc1 (5.9-rc1 is broken).
> 
> In all cases, I must confess it's quite pleased to post a series with diffstat
> like this...  Hopefully this won't break anyone but only to make everything
> better.
> 
> Please review, thanks.
> 
> [0] https://lore.kernel.org/lkml/CAHk-=wjn90-=s6MBerxTuP=-FVEZtR-LpoH9eenEQ3A-QfKTZw@mail.gmail.com
> [1] https://github.com/xzpeter/clibs/blob/master/bsd/mig_mon/mig_mon.c
> [2] https://github.com/LLNL/umap-apps/blob/develop/src/umapsort/umapsort.cpp
> [3] https://lore.kernel.org/lkml/CAHk-=wh0syDtNzt9jGyHRV0r1pVX5gkdJWdenwmvy=dq0AL5mA@mail.gmail.com
> [4] https://lore.kernel.org/lkml/CAHk-=wj5Oyg0LeAxSw_vizerm=sLd=sHfcVecZMKPZn6kNbbXA@mail.gmail.com
> 
> Linus Torvalds (1):
>   mm: Trial do_wp_page() simplification
> 
> Peter Xu (3):
>   mm/ksm: Remove reuse_ksm_page()
>   mm/gup: Remove enfornced COW mechanism
>   mm: Add PGREUSE counter
> 
>  drivers/gpu/drm/i915/gem/i915_gem_userptr.c |  8 ---
>  include/linux/ksm.h                         |  7 ---
>  include/linux/vm_event_item.h               |  1 +
>  mm/gup.c                                    | 40 ++------------
>  mm/huge_memory.c                            |  7 +--
>  mm/ksm.c                                    | 25 ---------
>  mm/memory.c                                 | 60 +++++++--------------
>  mm/vmstat.c                                 |  1 +
>  8 files changed, 29 insertions(+), 120 deletions(-)
>