On Mon, Jan 11, 2021 at 4:42 PM Ian Kent <raven@xxxxxxxxxx> wrote: > > On Mon, 2021-01-11 at 15:04 +0800, Fox Chen wrote: > > On Mon, Jan 11, 2021 at 12:20 PM Ian Kent <raven@xxxxxxxxxx> wrote: > > > On Mon, 2021-01-11 at 11:19 +0800, Ian Kent wrote: > > > > On Wed, 2021-01-06 at 10:38 +0800, Fox Chen wrote: > > > > > Hi Ian, > > > > > > > > > > I am rethinking this problem. Can we simply use a global lock? > > > > > > > > > > In your original patch 5, you have a global mutex attr_mutex > > > > > to > > > > > protect attr, if we change it to a rwsem, is it enough to > > > > > protect > > > > > both > > > > > inode and attr while having the concurrent read ability? > > > > > > > > > > like this patch I submitted. ( clearly, I missed > > > > > __kernfs_iattrs > > > > > part, > > > > > but just about that idea ) > > > > > https://lore.kernel.org/lkml/20201207084333.179132-1-foxhlchen@xxxxxxxxx/ > > > > > > > > I don't think so. > > > > > > > > kernfs_refresh_inode() writes to the inode so taking a read lock > > > > will allow multiple processes to concurrently update it which is > > > > what we need to avoid. > > > > Oh, got it. I missed the inode part. my bad. :( > > > > > > It's possibly even more interesting. > > > > > > > > For example, kernfs_iop_rmdir() and kernfs_iop_mkdir() might > > > > alter > > > > the inode link count (I don't know if that would be the sort of > > > > thing > > > > they would do but kernfs can't possibly know either). Both of > > > > these > > > > functions rely on the VFS locking for exclusion but the inode > > > > link > > > > count is updated in kernfs_refresh_inode() too. > > > > > > > > That's the case now, without any patches. > > > > > > So it's not so easy to get the inode from just the kernfs object > > > so these probably aren't a problem ... > > > > IIUC only when dop->revalidate, iop->lookup being called, the result > > of rmdir/mkdir will be sync with vfs. > > Don't quite get what you mean here? > > Do you mean something like, VFS objects are created on user access > to the file system. Given that user access generally means path > resolution possibly followed by some operation. > > I guess those VFS objects will go away some time after the access > but even thought the code looks like that should happen pretty > quickly after I've observed that these objects stay around longer > than expected. There wouldn't be any use in maintaining a least > recently used list of dentry candidates eligible to discard. Yes, that is what I meant. I think the duration may depend on the current ram pressure. though not quite sure, I'm still digging this part of code. > > > > kernfs_node is detached from vfs inode/dentry to save ram. > > > > > > I'm not entirely sure what's going on in kernfs_refresh_inode(). > > > > > > > > It could be as simple as being called with a NULL inode because > > > > the dentry concerned is negative at that point. I haven't had > > > > time to look closely at it TBH but I have been thinking about it. > > > > um, It shouldn't be called with a NULL inode, right? > > > > inode->i_mode = kn->mode; > > > > otherwise will crash. > > Yes, you're right about that. > > > > > > Certainly this can be called without a struct iattr having been > > > allocated ... and given it probably needs to remain a pointer > > > rather than embedded in the node the inode link count update > > > can't easily be protected from concurrent updates. > > > > > > If it was ok to do the allocation at inode creation the problem > > > becomes much simpler to resolve but I thought there were concerns > > > about ram consumption (although I don't think that was exactly what > > > was said?). > > > > > > > you meant iattr to be allocated at inode creation time?? > > yes, I think so. it's due to ram consumption. > > I did, yes. > > The actual problem is dealing with multiple concurrent updates to > the inode link count, the rest can work. > > Ian > fox
