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SubjectRe: [PATCH 2/3] Improve fairness when locking the per-superblock s_anon list
On Thu, Nov 09 2017, Al Viro wrote:

> On Thu, Nov 09, 2017 at 11:52:48AM -0800, Linus Torvalds wrote:
>> Honestly, looking at the code, the whole s_anon thing seems entirely
>> broken. There doesn't even seem to be much reason for it. In pretty
>> much all cases, we could just hash the damn dentry,
>>
>> The only reason for actually having s_anon seems to be that we want
>> some per-superblock list of these unconnected dentries for
>> shrink_dcache_for_umount().
>>
>> Everything else would actually be *much* happier with just having the
>> dentry on the regular hash table. It would entirely get rid of this
>> stupid performance problem, and it would actually simplify all the
>> code elsewhere, because it would remove special cases like this
>>
>> if (unlikely(IS_ROOT(dentry)))
>> b = &dentry->d_sb->s_anon;
>> else
>> b = d_hash(dentry->d_name.hash);
>>
>> and just turn them into
>>
>> b = d_hash(dentry->d_name.hash);
>>
>> so I really wonder if we could just get rid of s_anon entirely.
>>
>> Yes, getting rid of s_anon might involve crazy things like "let's just
>> walk all the dentries at umount time", but honestly, that sounds
>> preferable. Especially if we can just then do something like
>>
>> - set a special flag in the superblock if we ever use __d_obtain_alias()
>
> Automatically set for a lot of NFS mounts (whenever you mount more than one
> tree from the same server, IIRC)...
>
>> - only scan all the dentries on umount if that flag is set.
>>
>> Hmm?
>
> That looks like a bloody painful approach, IMO. I'm not saying I like
> Neil's patch, but I doubt that "let's just walk the entire dcache on
> umount" is a good idea.
>
> I wonder if separating the d_obtain_alias() and d_obtain_root() would be
> a good idea; the former outnumber the latter by many orders of magnitude.
> The tricky part is that we could have a disconnected directory from
> d_obtain_alias() with children already connected to it (and thus normally
> hashed by d_splice_alias()) and fail to connect the whole thing to parent.
>
> That leaves us with an orphaned tree that might stick around past the
> time when we drop all references to dentries in it. And we want to
> get those hunted down and shot on umount. Could we
> * make s_anon hold d_obtain_root ones + orphans from such
> failed reconnects
> * make final dput() treat hashed IS_ROOT as "don't retain it"
> * have d_obtain_alias() put into normal hash, leaving the
> "move to s_anon" part to reconnect failures.
> * keep umount side of things unchanged.
>
> I agree that temporary insertions into ->s_anon are bogus; hell, I'm not
> even sure we want to put it on _any_ list initially - we want it to look
> like it's hashed, so we could set ->next to NULL and have ->pprev point
> to itself. Then normal case for d_obtain_alias() would not bother
> the hash chains at all at allocation time, then have it put into the
> right hash chain on reconnect. And on reconnect failure the caller
> would've moved it to orphan list (i.e. ->s_anon).

I looked back at the original bug report, and it was d_obtain_alias()
that was particularly suffering. As this holds two spinlocks while
waiting for the bl_lock, a delay can affect other code that doesn't
touch s_anon.
So improving d_obtain_alias() would be a good goal (as long as we don't
just move the problem of course).

It isn't only "reconnect failure" that leaves things on s_anon.
We normally don't bother trying to connect non-directories.
So if an NFS server is getting lots of read/write request without opens
or other pathname lookups, it could easily have lots of disconnected
files being repeatedly accessed. Keeping the dentries on d_anon means
we don't need to keep allocating new ones for every request.

So I'm not keen on dropping an IS_ROOT() dentry at final dput(), but
it might make sense to add the dentry to the per-fs list of IS_ROOT
dentries at that time.

One possible approach would be to use d_child rather than d_hash to link
together dentries that don't have a parent.
We could assign a random number to d_name.hash so it could appear to be
hashed without imposing on any one hash chain. We would still need a
spinlock in the superblock to manage the d_anon list that links the
d_child's together...
I might try to see how the code looks.

Thanks,
NeilBrown

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