Re: [PATCH RFC] fadvise: move active pages to inactive list with POSIX_FADV_DONTNEED

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

(2011/06/27 18:17), Pádraig Brady wrote:
> On 27/06/11 06:38, KOSAKI Motohiro wrote:
>>> Hmm, What if you do want to evict it from the cache for testing purposes?
>>> Perhaps this functionality should be associated with POSIX_FADV_NOREUSE?
>>> dd has been recently modified to support invalidating the cache for a file,
>>> and it uses POSIX_FADV_DONTNEED for that.
>>> http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=5f311553
>>
>> This change don't break dd. dd don't have a special privilege of file cache
>> dropping if it's also used by other processes.
>>
>> if you want to drop a cache forcely (maybe for testing), you need to use
>> /proc/sys/vm/drop_caches. It's ok to ignore other processes activity because
>> it's privilege operation.
> 
> Well the function and privileges are separate things.
> I think we've agreed that the new functionality is
> best associated with POSIX_FADV_NOREUSE,
> and the existing functionality with POSIX_FADV_DONTNEED.
> 
> BTW, I don't think privileges are currently enforced
> as I got root to cache a file here with:
>   # (time md5sum; sleep 100) < big.file
> And a normal user was able to uncache with:
>   $ dd iflag=nocache if=big.file count=0
> Anyway as said, this is a separate "issue".

I'm failed to see your point. Why does dd need to ignore other
process activity? If no other process, this patch doesn't change
any behavior. Isn't it?