linux-kernel - Re: [PATCH] proc/kpageflags: add KPF

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Message-Id: <20180216155752.4a17cfd41875911c79807585@linux-foundation.org>
Date:   Fri, 16 Feb 2018 15:57:52 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH] proc/kpageflags: add KPF_WAITERS

On Sun, 11 Feb 2018 13:36:41 +0300 Konstantin Khlebnikov <khlebnikov@...dex-team.ru> wrote:

> KPF_WAITERS indicates tasks are waiting for a page lock or writeback.
> This might be false-positive, in this case next unlock will clear it.

Well, kpageflags is full of potential false-positives.  Or do you think
this flag is especially vulnerable?

In other words, under what circumstances will we have KPF_WAITERS set
when PG_locked and PG-writeback are clear?

> This looks like worth information not only for kernel hacking.

Why?  What are the use-cases, in detail?  How are we to justify this
modification?

> In tool page-types in non-raw mode treat KPF_WAITERS without
> KPF_LOCKED and KPF_WRITEBACK as false-positive and hide it.

>  fs/proc/page.c                         |    1 +
>  include/uapi/linux/kernel-page-flags.h |    1 +
>  tools/vm/page-types.c                  |    7 +++++++

Please update Documentation/vm/pagemap.txt.