|| ||Andrew Morton <akpm-AT-linux-foundation.org> |
|| ||Andrea Righi <andrea-AT-betterlinux.com> |
|| ||Re: [PATCH v4 0/2] fadvise: move active pages to inactive list with
|| ||Tue, 28 Jun 2011 15:12:33 -0700|
|| ||Minchan Kim <minchan.kim-AT-gmail.com>,
Johannes Weiner <hannes-AT-cmpxchg.org>,
Hugh Dickins <hughd-AT-google.com>,
Marcus Sorensen <marcus-AT-bluehost.com>,
Rik van Riel <riel-AT-redhat.com>, Theodore
Shaohua Li <shaohua.li-AT-intel.com>,
=?ISO-8859-1?Q?=E1draig?= Brady <P-AT-draigBrady.com>,
<linux-mm-AT-kvack.org>, LKML <linux-kernel-AT-vger.kernel.org>|
|| ||Article, Thread
On Mon, 27 Jun 2011 15:29:19 +0200
Andrea Righi <firstname.lastname@example.org> wrote:
> There were some reported problems in the past about trashing page cache when a
> backup software (i.e., rsync) touches a huge amount of pages (see for example
> This problem has been almost fixed by the Minchan Kim's patch  and a proper
> use of fadvise() in the backup software. For example this patch set  has
> been proposed for inclusion in rsync.
> However, there can be still other similar trashing problems: when the backup
> software reads all the source files, some of them may be part of the actual
> working set of the system. When a POSIX_FADV_DONTNEED is performed _all_ pages
> are evicted from pagecache, both the working set and the use-once pages touched
> only by the backup software.
> With the following solution when POSIX_FADV_DONTNEED is called for an active
> page instead of removing it from the page cache it is added to the tail of the
> inactive list. Otherwise, if it's already in the inactive list the page is
> removed from the page cache. Pages mapped by other processes or unevictable
> pages are not touched at all.
> In this way if the backup was the only user of a page, that page will be
> immediately removed from the page cache by calling POSIX_FADV_DONTNEED. If the
> page was also touched by other processes it'll be moved to the inactive list,
> having another chance of being re-added to the working set, or simply reclaimed
> when memory is needed.
So if an application touches a page twice and then runs
POSIX_FADV_DONTNEED, that page will now not be freed.
That's a big behaviour change. For many existing users
POSIX_FADV_DONTNEED simply doesn't work any more!
I'd have thought that adding a new POSIX_FADV_ANDREA would be safer
The various POSIX_FADV_foo's are so ill-defined that it was a mistake
to ever use them. We should have done something overtly linux-specific
and given userspace more explicit and direct pagecache control.
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