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    SubjectRe: [RFC] [PATCH] drop_pagecache syscall
    On Wed, Apr 27, 2011 at 05:47, Andrea Righi wrote:
    > On Wed, Apr 27, 2011 at 05:10:41AM -0400, Mike Frysinger wrote:
    >> On Wed, Apr 27, 2011 at 05:01, Andrea Righi wrote:
    >> > On Wed, Apr 27, 2011 at 10:14:53AM +1000, Dave Chinner wrote:
    >> >> On Tue, Apr 26, 2011 at 11:35:27PM +0200, Andrea Righi wrote:
    >> >> > This functionality can be used by all the applications that want to have a
    >> >> > better control over the page cache management (for example to immediately drop
    >> >> > pages that for sure will not be reused in the near future, without calling
    >> >> > posix_fadvise() for all the files they've touched), or to provide a more fine
    >> >> > grained debugging feature usable by the filesystem benchmarks.
    >> >> >
    >> >> > The system call does not require root privileges and it can be called by any
    >> >> > unprivileged application. For example, we can write a userspace tool to run
    >> >> > something like this:
    >> >> >
    >> >> >   $ drop-pagecache /path/file_or_dir
    >> >>
    >> >> That's a potential DOS vector, I think. Drop the pagecache in a hard
    >> >> loop on the root fs of a busy server and watch it crawl...
    >> >
    >> > Yes, probably we could allow only the CAP_SYS_ADMIN tasks to execute
    >> > this syscall.
    >>
    >> if /proc/sys/vm/drop_caches has any checks other than file permission
    >> checks (i.e. UID==0), it'd probably be better to copy those rather
    >> than picking something different.
    >
    > ok, what about checking current_euid() == 0?

    that's not what i meant. if the drop_caches file already has certain
    cap checks/whatever in place, let's use those. if it doesnt, then
    picking a cap level as you proposed makes sense.
    -mike
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