Re: invalidate the buffer heads of a block device

[Trond Myklebust <trond.myklebust@primarydata.com>]

On Tue, Sep 30, 2014 at 4:53 PM, Zach Brown <zab@zabbo.net> wrote:
> On Tue, Sep 30, 2014 at 12:48:45PM +0200, Jan Kara wrote:
>> On Tue 30-09-14 10:11:32, Thanos Makatos wrote:
>> >
>> > Regarding extending the ioctl to invalidate the page cache, do you  have
>> > any suggestions where I could start looking?
>>   You just need to call invalidate_inode_pages2(). That is going to do all
>> you need.
>>
>> > Would such a new ioctl have any chance to be accepted upstream?
>>   I believe a possibility for a file to be fully flushed from page cache is
>> useful at times and if you present well your usecase there are reasonable
>> chances it will get accepted upstream.
>
> Agreed, this seems reasonable.  How many times have we all dropped our
> entire cache just 'cause we didn't have a more precise tool?
>
> $ grep -ri drop_caches xfstests/
> xfstests/src/fsync-tester.c:    if ((fd = open("/proc/sys/vm/drop_caches", O_WRONLY)) < 0) {
> xfstests/src/stale_handle.c:    system("echo 3 > /proc/sys/vm/drop_caches");
> xfstests/common/quota:  echo 3 > /proc/sys/vm/drop_caches
>
> The last one even says:
>
>         # XXX: really need an ioctl instead of this big hammer
>         echo 3 > /proc/sys/vm/drop_caches
>
> :)
>

It would definitely be useful for NFS, however we'd want the option of
clearing the cached metadata too (acls, mode bits, owner/group owner,
etc.)

-- 
Trond Myklebust

Linux NFS client maintainer, PrimaryData

trond.myklebust@primarydata.com