Re: [RFC Patch] fs: implement per-file drop caches

[Cong Wang <amwang@redhat.com>]

On Thu, 2012-05-31 at 02:30 -0400, KOSAKI Motohiro wrote:
> (5/31/12 2:20 AM), Cong Wang wrote:
> > On Wed, 2012-05-30 at 16:14 +0100, Pádraig Brady wrote:
> >> On 05/30/2012 02:38 PM, Cong Wang wrote:
> >>> This is a draft patch of implementing per-file drop caches.
> >>>
> >>> It introduces a new fcntl command  F_DROP_CACHES to drop
> >>> file caches of a specific file. The reason is that currently
> >>> we only have a system-wide drop caches interface, it could
> >>> cause system-wide performance down if we drop all page caches
> >>> when we actually want to drop the caches of some huge file.
> >>
> >> This is useful functionality.
> >> Though isn't it already provided with POSIX_FADV_DONTNEED?
> >
> > Thanks for teaching this!
> >
> > However, from the source code of madvise_dontneed() it looks like it is
> > using a totally different way to drop page caches, that is to invalidate
> > the page mapping, and trigger a re-mapping of the file pages after a
> > page fault. So, yeah, this could probably drop the page caches too (I am
> > not so sure, haven't checked the code in details), but with my patch, it
> > flushes the page caches directly, what's more, it can also prune
> > dcache/icache of the file.
> 
> madvise should work. I don't think we need duplicate interface. Moreomover
> madvise(2) is cleaner than fcntl(2).
> 

I think madvise(DONTNEED) attacks the problem in a different approach,
it munmaps the file mapping and by the way drops the page caches, my
approach is to drop the page caches directly similar to what sysctl
drop_caches.

What about private file mapping? Could madvise(DONTNEED) drop the page
caches too even when the other process is doing the same private file
mapping? At least my patch could do this.

I am not sure if fcntl() is a good interface either, this is why the
patch is marked as RFC. :-D

Thanks!

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