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