On 29/06/11 00:03, Andrew Morton wrote:

On Wed, 29 Jun 2011 00:56:45 +0200
Andrea Righi [off-list ref] wrote:

quoted

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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.

quoted

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.

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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,
Padraig.

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