Re: [RFC] [PATCH] drop_pagecache syscall

[Mike Frysinger]

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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