On Mon, 27 Jun 2011 15:29:19 +0200 Andrea Righi <andrea@xxxxxxxxxxxxxxx> 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 > [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! I'd have thought that adding a new POSIX_FADV_ANDREA would be safer than this. 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>