Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects

From: Tobin C. Harding
Date: Thu Apr 11 2019 - 01:06:27 EST

On Thu, Apr 11, 2019 at 05:47:46AM +0100, Al Viro wrote:
> On Thu, Apr 11, 2019 at 12:48:21PM +1000, Tobin C. Harding wrote:
> > Oh, so putting entries on a shrink list is enough to pin them?
> Not exactly pin, but __dentry_kill() has this:
> if (dentry->d_flags & DCACHE_SHRINK_LIST) {
> dentry->d_flags |= DCACHE_MAY_FREE;
> can_free = false;
> }
> spin_unlock(&dentry->d_lock);
> if (likely(can_free))
> dentry_free(dentry);
> and shrink_dentry_list() - this:
> if (dentry->d_lockref.count < 0)
> can_free = dentry->d_flags & DCACHE_MAY_FREE;
> spin_unlock(&dentry->d_lock);
> if (can_free)
> dentry_free(dentry);
> continue;
> so if dentry destruction comes before we get around to
> shrink_dentry_list(), it'll stop short of dentry_free() and mark it for
> shrink_dentry_list() to do just dentry_free(); if it overlaps with
> shrink_dentry_list(), but doesn't progress all the way to freeing,
> we will
> * have dentry removed from shrink list
> * notice the negative ->d_count (i.e. that it has already reached
> __dentry_kill())
> * see that __dentry_kill() is not through with tearing the sucker
> apart (no DCACHE_MAY_FREE set)
> ... and just leave it alone, letting __dentry_kill() do the rest of its
> thing - it's already off the shrink list, so __dentry_kill() will do
> everything, including dentry_free().
> The reason for that dance is the locking - shrink list belongs to whoever
> has set it up and nobody else is modifying it. So __dentry_kill() doesn't
> even try to remove the victim from there; it does all the teardown
> (detaches from inode, unhashes, etc.) and leaves removal from the shrink
> list and actual freeing to the owner of shrink list. That way we don't
> have to protect all shrink lists a single lock (contention on it would
> be painful) and we don't have to play with per-shrink-list locks and
> all the attendant headaches (those lists usually live on stack frame
> of some function, so just having the lock next to the list_head would
> do us no good, etc.). Much easier to have the shrink_dentry_list()
> do all the manipulations...
> The bottom line is, once it's on a shrink list, it'll stay there
> until shrink_dentry_list(). It may get extra references after
> being inserted there (e.g. be found by hash lookup), it may drop
> those, whatever - it won't get freed until we run shrink_dentry_list().
> If it ends up with extra references, no problem - shrink_dentry_list()
> will just kick it off the shrink list and leave it alone.
> Note, BTW, that umount coming between isolate and drop is not a problem;
> it call shrink_dcache_parent() on the root. And if shrink_dcache_parent()
> finds something on (another) shrink list, it won't put it to the shrink
> list of its own, but it will make note of that and repeat the scan in
> such case. So if we find something with zero refcount and not on
> shrink list, we can move it to our shrink list and be sure that its
> superblock won't go away under us...

Man, that was good to read. Thanks for taking the time to write this.