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

[Cong Wang <amwang@redhat.com>]

On Thu, 2012-05-31 at 15:09 -0400, KOSAKI Motohiro wrote:
> (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, 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.
> 
> 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.
> 

Ok, then this patch is not a dup of madvise(DONTNEED).

> 
> > 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.
> 

Yeah, at least John Stoffel expressed his interests on this, as a
sysadmin. So I believe there are some people need it.

Now the problem is that I don't find a proper existing utility to patch,
maybe Pádraig has any hints on this? Could this feature be merged into
some core utility? Or I have to write a new utility for this?

Thanks.

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