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


    > 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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