On Wed, 2021-01-13 at 15:00 +0800, Fox Chen wrote: > On Wed, Jan 13, 2021 at 1:17 PM Ian Kent <raven@xxxxxxxxxx> wrote: > > On Mon, 2021-01-11 at 17:02 +0800, Fox Chen wrote: > > > 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. > > > > Umm ... maybe I've been trying to do this in the wrong place all > > along. > > > > You know the inode i_lock looks like the sensible thing to use to > > protect these updates. > > > > Something like this for that last patch should work: > > > > kernfs: use i_lock to protect concurrent inode updates > > > > From: Ian Kent <raven@xxxxxxxxxx> > > > > The inode operations .permission() and .getattr() use the kernfs > > node > > write lock but all that's needed is to keep the rb tree stable > > while > > updating the inode attributes as well as protecting the update > > itself > > against concurrent changes. > > > > And .permission() is called frequently during path walks and can > > cause > > quite a bit of contention between kernfs node opertations and path > > walks when the number of concurrant walks is high. > > > > To change kernfs_iop_getattr() and kernfs_iop_permission() to take > > the rw sem read lock instead of the write lock an addtional lock is > > needed to protect against multiple processes concurrently updating > > the inode attributes and link count in kernfs_refresh_inode(). > > > > The inode i_lock seems like the sensible thing to use to protect > > these > > inode attribute updates so use it in kernfs_refresh_inode(). > > > > Signed-off-by: Ian Kent <raven@xxxxxxxxxx> > > --- > > fs/kernfs/inode.c | 10 ++++++---- > > 1 file changed, 6 insertions(+), 4 deletions(-) > > > > diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c > > index ddaf18198935..e26fa5115821 100644 > > --- a/fs/kernfs/inode.c > > +++ b/fs/kernfs/inode.c > > @@ -171,6 +171,7 @@ static void kernfs_refresh_inode(struct > > kernfs_node *kn, struct inode *inode) > > { > > struct kernfs_iattrs *attrs = kn->iattr; > > > > + spin_lock(inode->i_lock); > > inode->i_mode = kn->mode; > > if (attrs) > > /* > > @@ -181,6 +182,7 @@ static void kernfs_refresh_inode(struct > > kernfs_node *kn, struct inode *inode) > > > > if (kernfs_type(kn) == KERNFS_DIR) > > set_nlink(inode, kn->dir.subdirs + 2); > > + spin_unlock(inode->i_lock); > > } > > > > int kernfs_iop_getattr(const struct path *path, struct kstat > > *stat, > > @@ -189,9 +191,9 @@ int kernfs_iop_getattr(const struct path *path, > > struct kstat *stat, > > struct inode *inode = d_inode(path->dentry); > > struct kernfs_node *kn = inode->i_private; > > > > - down_write(&kernfs_rwsem); > > + down_read(&kernfs_rwsem); > > kernfs_refresh_inode(kn, inode); > > - up_write(&kernfs_rwsem); > > + up_read(&kernfs_rwsem); > > > > generic_fillattr(inode, stat); > > return 0; > > @@ -281,9 +283,9 @@ int kernfs_iop_permission(struct inode *inode, > > int mask) > > > > kn = inode->i_private; > > > > - down_write(&kernfs_rwsem); > > + down_read(&kernfs_rwsem); > > kernfs_refresh_inode(kn, inode); > > - up_write(&kernfs_rwsem); > > + up_read(&kernfs_rwsem); > > > > return generic_permission(inode, mask); > > } > > > > It looks good on my local machine, let me test my benchmark on a big > machine. :) > > Also, I wonder why i_lock?? what if I use a local spin_lock, will > there be any difference??? I think that amounts to using a global lock (ie. static) not a per-object lock which is needed to reduce contention. > > static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode > *inode) > { > struct kernfs_iattrs *attrs = kn->iattr; > static DEFINE_SPINLOCK(inode_lock); > > spin_lock(&inode_lock); > inode->i_mode = kn->mode; > if (attrs) > /* > * kernfs_node has non-default attributes get them > from > * persistent copy in kernfs_node. > */ > set_inode_attr(inode, attrs); > > if (kernfs_type(kn) == KERNFS_DIR) > set_nlink(inode, kn->dir.subdirs + 2); > spin_unlock(&inode_lock); > } > > > > thanks, > fox
