On Tue, Jun 28, 2011 at 03:12:33PM -0700, Andrew Morton wrote:

On Mon, 27 Jun 2011 15:29:19 +0200
Andrea Righi [off-list ref] wrote:

quoted

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
[1]).
    
This problem has been almost fixed by the Minchan Kim's patch [2] and a proper
use of fadvise() in the backup software. For example this patch set [3] 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!

Yes. This is the main concern that was raised by Padraig.

I'd have thought that adding a new POSIX_FADV_ANDREA would be safer
than this.

Actually Jerry (in cc) proposed
POSIX_FADV_IDONTNEEDTHISBUTIFSOMEBODYELSEDOESTHENDONTTOUCHIT in a
private email. :)

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.

That would give us the possibility to implement a wide range of
different operations (drop, drop if used once, add to the active list,
add to the inactive list, etc..). Some users always complain that they
would like to have a better control over the page cache from userspace.

-Andrea

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