On Wed, Jun 29, 2011 at 12:20:22PM +0100, Pádraig Brady wrote: > On 29/06/11 00:03, Andrew Morton wrote: > > On Wed, 29 Jun 2011 00:56:45 +0200 > > Andrea Righi <andrea@xxxxxxxxxxxxxxx> wrote: > > > >>>> > >>>> 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 P__draig. > >> > >>> > >>> 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. :) > > > > Sounds good. Needs more underscores though. > > > >>> > >>> > >>> 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. > > > > Well, I'd listen to proposals ;) > > > > One thing we must be careful about is to not expose things like "active > > list" to userspace. linux-4.5 may not _have_ an active list, and its > > implementors would hate us and would have to jump through hoops to > > implement vaguely compatible behaviour in the new scheme. > > > > So any primitives which are exposed should be easily implementable and > > should *make sense* within any future scheme... > > Agreed. > > In fairness to posix_fadvise(), I think it's designed to > specify hints for the current process' use of data > so that it can get at it more efficiently and also be > allow the system to manipulate cache more efficiently. > I.E. it's not meant for direct control of the cache. > > That being said, existing use has allowed this, > and it would be nice not to change without consideration. > > I've mentioned how high level cache control functions > might map to the existing FADV knobs here: > > http://marc.info/?l=linux-kernel&m=130917619416123&w=2 > > cheers, > Pádraig. OK, your proposal seems a good start to implement a better cache control interface. Basically you're proposing to provide the following operations: 1. DROP 2. DROP if used once 3. ADD 4. ADD if there's space I would also add for sure: 5. ADD and will use once Some of them are already implemented by the available fadvise() operations, like 1 (POSIX_FADV_DONTNEED) and 3 (POSIX_FADV_WILLNEED). Option 5 can be mapped to POSIX_FADV_NOREUSE, but it's not yet implemented. I need to think a little bit more about all of this. I'll try to post a new RFC, proposing the list of high-level operations to implement the better page cache control from userspace. Suggestions, comments, ideas are always welcome. Thanks, -Andrea -- 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>