On (06/04/15 10:53), Minchan Kim wrote: > > hm, sounds interesting, but I think it will end up being tricky. > > > > zram_remove() will be called from device's sysfs node (now we call it from > > zram_control sysfs class node, makes a huge difference). sysfs locks the node > > until node's read/write handler returns back, so zram_remove() will be called > > with lock(s_active#XXX) being locked (we had a lockdep splat with these locks > > recently), while zram_remove()->sysfs_remove_group() will once again attempt > > to lock this node (the very same lock(s_active#XXX)). in other words, we cannot > > fully remove zram device from its sysfs attr. and I don't want to add any bool > > flags to zram_remove() and zram_add() indicating that this is a "partial" device > > remove: don't delete device's sysfs group in remove() and don't create it in add(). > > > > > > doing reset from zram_control is easy, for sure: > > lock idr mutex, > > do zram_remove() and zram_add() > > unlock idr lock. > > > > `echo ID > /sys/.../zram_control/reset` > > > > no need to modify remove()/add() -- idr will pick up just released idx, > > so device_id will be preserved. but it'll be hard to drop the per-device > > `reset` attr and to make it a zram_control attr. things would have been > > much simpler if all of zram users were also zramctl users. zramctl, from > > this point of view, lets us change zram interfaces easily -- we merely need > > to teach/modify zramctl, the rest is transparent. > > Thanks for the looking. > Fair enough. > > So you mean you don't want to add any bool flags. Instead, you want to move > reset interface into /sys/.../zram_control/reset and it would be transparent > if everyone doesn't use raw interface. I just described the ideal case -- moving reset to zram_control. which is very much unlikely to happen. even if zramX/reset will become a symlink to zram_control/reset user still will have to supply a device_id. it's too late to change this, unfortunately. > Somethings I have in mind. > > We should change old interface(ie, /sys/block/zram0/reset) by just > *implementation difficulty* which is just adding a bool flag? > IMO, it's not a good reason to change old interface. > I prefer adding a bool flag if it can meet our goal entirely. well, we can add it. but it's hacky and tricky. having a clear "zram_add(void)/zram_remove(void)" vs. "zram_add(bool partial)/zram_remove(bool partial)". apart from that, zram_add() will introduce additional 4 places where we can fail to re-create the device: -- zram = kzalloc(sizeof(struct zram), GFP_KERNEL); -- ret = idr_alloc(&zram_index_idr, zram, 0, 0, GFP_KERNEL); -- queue = blk_alloc_queue(GFP_KERNEL); -- zram->disk = alloc_disk(1); so, we don't destroy and create zram's sysfs_group. which means that we better not kfree() and kzalloc() zram pointer, otherise we still need to set up &disk_to_dev(zram->disk)->kobj. so 'bool partial' flag will now also make zram kfree()/kmalloc() optional. if we have kfree()/kmalloc() optional, then we probably should keep idr allocation optional as well. iow, optional idr_alloc/idr_remove(). which sort of turns zram_add()/zram_remove() into a hell. I need to think about it more. > Another thing I repeated several times is that we cannot guarantee > every users in the world will use zramctl forever so we should > be careful to change interface even though a userland tool becomes > popular. no, of course I'm not saying that everyone is using zramctl nor I count on it, zram is simply ~4 years older than zramctl. *things would have been much simpler if* ... -ss -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>