linux-kernel - Re: [RFC PATCH] mm: fadvise: avoid fadvise for fs without backing device

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Message-Id: <20170825144957.5d99dad605fed1dc2550d25c@linux-foundation.org>
Date:   Fri, 25 Aug 2017 14:49:57 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: fadvise: avoid fadvise for fs without backing
 device

On Thu, 17 Aug 2017 18:20:17 -0700 Shakeel Butt <shakeelb@...gle.com> wrote:

> +linux-mm, linux-kernel
> 
> On Thu, Aug 17, 2017 at 6:10 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
> > The fadvise() manpage is silent on fadvise()'s effect on
> > memory-based filesystems (shmem, hugetlbfs & ramfs) and pseudo
> > file systems (procfs, sysfs, kernfs). The current implementaion
> > of fadvise is mostly a noop for such filesystems except for
> > FADV_DONTNEED which will trigger expensive remote LRU cache
> > draining. This patch makes the noop of fadvise() on such file
> > systems very explicit.
> >
> > However this change has two side effects for ramfs and one for
> > tmpfs. First fadvise(FADV_DONTNEED) can remove the unmapped clean
> > zero'ed pages of ramfs (allocated through read, readahead & read
> > fault) and tmpfs (allocated through read fault). Also
> > fadvise(FADV_WILLNEED) on create such clean zero'ed pages for
> > ramfs.

That sentence makes no sense.  I assume "fadvise(FADV_WILLNEED) will
create"?