Re: Drop cache has no effect?

Hello,

recently I picked up knowledge of /proc/sys/vm/drop_caches
(http://lkml.org/lkml/2006/8/4/95)

It does not always work right away:

(/U is a vfat, that is, permissions are back to 755 as soon as the caches
are gone)
14:51 gwdg-wb04A:/U # chmod 644 *
14:51 gwdg-wb04A:/U # sync
14:51 gwdg-wb04A:/U # echo 2 >/proc/sys/vm/drop_caches
14:51 gwdg-wb04A:/U # l
total 50713
drwxr-xr-x 3 jengelh users 2048 2006-08-29 14:48 .
drwxr-xr-x 22 root root 4096 2006-08-25 14:00 ..
drw-r--r-- 2 jengelh users 2048 2006-08-29 13:55 as
-rw-r--r-- 1 jengelh users 13806629 2006-08-29 14:00 all-20060611.tar.bz2
-rw-r--r-- 1 jengelh users 37816633 2006-07-28 19:25
inkscape-0.44-2.guru.suse101.i686.rpm
-rw-r--r-- 1 jengelh users 297243 2006-08-15 01:13
vmware-any-any-update104.tar.gz

Remains 644.

That would be a vfat problem - the changed permission bits weren't written
back to disk, so when you re-read them from disk (or, more likely, from
blockdev pagecache) they came back with the original values.

Yes, that's _intended_.

Fact:
If you chmod 644 some files on vfat, then unmount and mount it again, they show
up as 755 again. That is ok.

Observation:
Dropping the cache does not imply the 644->755 change observed on unmount.

Conclusion:
Caches not dropped.


Does vfat even have the ability to store the seven bits? Don't think so?
If not, permitting the user to change them in icache but not being to write
them out to permanent store seems rather bad behaviour.

It is, I think, for compat reasons. Who knows how many apps don't expect chmod
to fail when they know you are the owner.



thanks,
Jan Engelhardt
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