Details
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Bug
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Resolution: Fixed
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Major
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Lustre 2.1.3, Lustre 2.1.4
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3
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6116
Description
We have a lot of nodes with a large amount of unreclaimable memory (over 4GB). Whatever we try to do (manually shrinking the cache, clearing lru locks, ...) the memory can't be recovered. The only way to get the memory back is to umount the lustre filesystem.
After some troubleshooting, I was able to wrote a small reproducer where I just open(2) then close(2) files in O_RDWR (my reproducer use to open thousand of files to emphasize the issue).
Attached 2 programs :
- gentree.c (cc -o gentree gentree.c -lpthread) to generate a tree of known files (no need to use readdir in reproducer.c)
- reproducer.c (cc -o reproducer reproduver.c -lpthread) to reproduce the issue.
The macro BASE_DIR has to be adjust according the local cluster configuration (you should provide the name of a directory located on a lustre filesystem).
There is no link between the 2 phases as rebooting the client between gentree & reproducer does't avoid the problem. Running gentree (which open as much files as reproducer) doesn't show the issue.
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Mike, your solution looks fine to me, I'll update the patch in this way soon. Thanks.
Niu, in fact we don't need to wait for commit in case of closed open (no create) and exactly that case causes this bug with unreclaimable space. And I don't see why server help is needed here - client knows there was close and knows this is non-create open - that is enough to make decision to drop request from replay queue. I am not sure though how easy to distinguish non-create case from OPEN-CREATE, at first sign we need to check disposition flag for DISP_OPEN_CREATE bit. So possible solution can be:
1) after open reply check disposition for DISP_OPEN_CREATE bit and save that information in md_open_data, OR just take disposition from already saved mod_open_req during mdc_close()
2) in mdc_close() there is already mod->mod_open_req->rq_replay is set to 0, we set also mod_open_req->rq_commit_nowait or any other new flag for non-create open.
3) in ptlrpc_free_committed() check that rq_commit_nowait flag and free such request immediately no matter what transno it has.
Will that works? Am I missing something?
yes, I also remember we discussed a way to implement openhandle as a LDLM lock and let LDLM to re-enqueue locks at recovery.
Niu, exactly, and I propose to make that 'existing code' able to drop closed open regardless its transno because it doesn't make sense after close. Current solution is still based on hacking server side in various ways. In fact that can be solved at client side, just by letting closed OPENs to be dropped despite their transno.
Mike, I think there is no way to achieve this without server side changes, I can think of two ways so far:
1. Server treats open/close as committed transactions, and returns client both last committed transno & last real transno (on-disk transno), client drops committed open & close request immediately after close. That's what I did in my patch.
2. Server assigns no transno for open/close, and client open-replay mechanism must be adapted to this change (like Siyao mentioned in the review comment: track open handle in fs layer, and rebuild request when replay the open, and some other changes to open, close, open lock code could be required)
The second solution looks cleaner to me, but it requires more code changes, and it'll be little tricky to handle open-create & open differently on client side.
Niu, exactly, and I propose to make that 'existing code' able to drop closed open regardless its transno because it doesn't make sense after close. Current solution is still based on hacking server side in various ways. In fact that can be solved at client side, just by letting closed OPENs to be dropped despite their transno.
Andreas, the existing code can only drop open/close request when the last_committed seen by client is bumped, no matter if it's an open_create or not.
If the request is not doing a create, couldn't both the open and close RPC be dropped at this time, regardless of the transno?
Mike, right. I mentioned this (the open request can only be freed when the last_committed on client is bumped) in previous comment, and it's same for the close request. Adding another flag for checking open/close request might be work, what about my solution in http://review.whamcloud.com/6665 ? Could you review it? Thanks.
Niu, Andreas, this become more and more complex as I can see. The problem we are trying to solve is that client keeps closed open requests in replay queue, right? Meanwhile the client itself wants them to be dropped from that queue, see mdc_close():
/* We no longer want to preserve this open for replay even * though the open was committed. b=3632, b=3633 */ spin_lock(&mod->mod_open_req->rq_lock); mod->mod_open_req->rq_replay = 0; spin_unlock(&mod->mod_open_req->rq_lock);
So the problem is that request is not dropped when client ask to do that. That is because of last_committed check which is the only mechanism to drop request and that means for me we just need to add another one to drop request from replay queue regardless its transno. E.g. rq_closed_open flag can be added and checked to drop request. That could be much simpler. Did I miss something and there are other cases we are trying to solve?
Niu, I am not so sure it will be easy to implement, this is just possible way to go but if that will work that would be good.