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

[KOSAKI Motohiro]

(5/31/12 8:11 AM), Cong Wang wrote:
> 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, PÃdraig 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.

Right. But a process can makes another mappings if a process have enough
permission. and if it doesn't, a process shouldn't be able to drop a shared
cache.


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

But, if you can find certain usecase, I'm not against anymore.


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