Re: [PATCH] ceph: Update the pages in fscache in writepages() path

From: Milosz Tanski
Date: Tue Nov 19 2013 - 21:30:11 EST

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In reply to: Yan, Zheng: "Re: [PATCH] ceph: Update the pages in fscache in writepages() path"
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Yan,

I'll use this trick next time around. I did dump the kernel stacks for
my process. 4 threads were blocked on SYS_newfstat (and the mds
request further up the stack).

I ended up restarting MDS after a few hours of trying to track it
down. It resolved it self following that.

This machine is running a pretty recent kernel on there -- 3.12 +
merged testing ceph -- the other machines are running a slightly older
3.12-rc. I have observed the issue previously on random nodes
infrequently for months (maybe once a week or two).

Thanks again,
- Milosz

On Tue, Nov 19, 2013 at 8:18 PM, Yan, Zheng <ukernel@xxxxxxxxx> wrote:
> On Wed, Nov 20, 2013 at 12:05 AM, Milosz Tanski <milosz@xxxxxxxxx> wrote:
>> Yan and Sage,
>>
>> I've ran into this issue again on my test cluster. The client hangs
>> all requests for a particular inode, I did a dump cache to see what's
>> going... but I don't understand to enough to be able to read this line
>> well enough.
>>
>> Can you guys help me read this, so I can further track down and
>> hopefully fix this issue.
>>
>> [inode 10000346eed [2,head]
>> /petabucket/beta/17511b3d12466609785b6a0e34597431721d177240371c0a1a4e347a1605381b/advertiser_id.dict
>> auth v214 ap=5+0 dirtyparent s=925 n(v0 b925 1=1+0) (ifile sync->mix)
>> (iversion lock) cr={59947=0-4194304@1}
>> caps={59947=pAsLsXsFr/pAsxXsxFxwb@26,60001=pAsLsXsFr/-@1,60655=pAsLsXsFr/pAsLsXsFscr/pFscr@36}
>> | ptrwaiter=0 request=4 lock=1 caps=1 dirtyparent=1 dirty=1 waiter=1
>> authpin=1 0x17dd6b70]
>>
>> root@bnode-16a1ed7d:~# cat
>> /sys/kernel/debug/ceph/e23a1bfc-8328-46bf-bc59-1209df3f5434.client60655/mdsc
>> 15659 mds0 getattr #10000346eed
>> 15679 mds0 getattr #10000346eed
>> 15710 mds0 getattr #10000346eed
>> 15922 mds0 getattr #10000346eed
>
> which kernel do you use? is there any blocked process (echo w >
> /proc/sysrq-trigger) on client.60655 ? 3.12 kernel contains few fixes
> for similar hang.
>
> Regards
> Yan, Zheng
>
>>
>> On Wed, Nov 6, 2013 at 10:01 AM, Yan, Zheng <ukernel@xxxxxxxxx> wrote:
>>> On Wed, Nov 6, 2013 at 9:41 PM, Milosz Tanski <milosz@xxxxxxxxx> wrote:
>>>> Sage,
>>>>
>>>> I think the incrementing version counter on the whole is a neater
>>>> solution then using size and mtime. If nothing else it's more explicit
>>>> in the the read cache version. With what you suggested plus additional
>>>> changes to the open code (where the cookie gets created) the
>>>> write-through scenario should be correct.
>>>>
>>>> Sadly, my understanding of the MDS protocol is still not great. So
>>>> when doing this in the first place I erred on the side of using what
>>>> was already in place.
>>>>
>>>> In a kind of un-related question. Is there a debug hook in the kclient
>>>> (or MDS for that matter) to dump the current file inodes (names) with
>>>> issues caps and to which hosts. This would be very helpful for
>>>> debugging, since from time to time I see a one of the clients get
>>>> stuck in getattr (via mdsc debug log).
>>>>
>>>
>>> "ceph mds tell \* dumpcache" dump the mds cache to a file. the dump
>>> file contains caps information.
>>>
>>> Regards
>>> Yan, Zheng
>>>
>>>> Thanks,
>>>> - Milosz
>>>>
>>>> On Tue, Nov 5, 2013 at 6:56 PM, Sage Weil <sage@xxxxxxxxxxx> wrote:
>>>>> On Tue, 5 Nov 2013, Milosz Tanski wrote:
>>>>>> Li,
>>>>>>
>>>>>> First, sorry for the late reply on this.
>>>>>>
>>>>>> Currently fscache is only supported for files that are open in read
>>>>>> only mode. I originally was going to let fscache cache in the write
>>>>>> path as well as long as the file was open in with O_LAZY. I abandoned
>>>>>> that idea. When a user opens the file in O_LAZY we can cache things
>>>>>> locally with the assumption that the user will care of the
>>>>>> synchronization in some other manner. There is no way of invalidating
>>>>>> a subset of the pages in object cached by fscache, there is no way we
>>>>>> can make O_LAZY work well.
>>>>>>
>>>>>> The ceph_readpage_to_fscache() in writepage has no effect and it
>>>>>> should be removed. ceph_readpage_to_fscache() calls cache_valid() to
>>>>>> see if it should perform the page save, and since the file can't have
>>>>>> a CACHE cap at the point in time it doesn't do it.
>>>>>
>>>>> (Hmm, Dusting off my understanding of fscache and reading
>>>>> fs/ceph/cache.c; watch out!) It looks like cache_valid is
>>>>>
>>>>> static inline int cache_valid(struct ceph_inode_info *ci)
>>>>> {
>>>>> return ((ceph_caps_issued(ci) & CEPH_CAP_FILE_CACHE) &&
>>>>> (ci->i_fscache_gen == ci->i_rdcache_gen));
>>>>> }
>>>>>
>>>>> and in the FILE_EXCL case, the MDS will issue CACHE|BUFFER caps. But I
>>>>> think the aux key (size+mtime) will prevent any use of the cache as soon
>>>>> as the first write happens and mtime changes, right?
>>>>>
>>>>> I think that in order to make this work, we need to fix/create a
>>>>> file_version (or something similar) field in the (mds) inode_t to have
>>>>> some useful value. I.e., increment it any time
>>>>>
>>>>> - a different client/writer comes along
>>>>> - a file is modified by the mds (e.g., truncated or recovered)
>>>>>
>>>>> but allow it to otherwise remain the same as long as only a single client
>>>>> is working with the file exclusively. This will be more precise than the
>>>>> (size, mtime) check that is currently used, and would remain valid when a
>>>>> single client opens the same file for exclusive read/write multiple times
>>>>> but there are no other intervening changes.
>>>>>
>>>>> Milosz, if that were in place, is there any reason not to wire up
>>>>> writepage and allow the fscache to be used write-through?
>>>>>
>>>>> sage
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> - Milosz
>>>>>>
>>>>>> On Thu, Oct 31, 2013 at 11:56 PM, Li Wang <liwang@xxxxxxxxxxxxxxx> wrote:
>>>>>> > Currently, the pages in fscache only are updated in writepage() path,
>>>>>> > add the process in writepages().
>>>>>> >
>>>>>> > Signed-off-by: Min Chen <minchen@xxxxxxxxxxxxxxx>
>>>>>> > Signed-off-by: Li Wang <liwang@xxxxxxxxxxxxxxx>
>>>>>> > Signed-off-by: Yunchuan Wen <yunchuanwen@xxxxxxxxxxxxxxx>
>>>>>> > ---
>>>>>> > fs/ceph/addr.c | 8 +++++---
>>>>>> > 1 file changed, 5 insertions(+), 3 deletions(-)
>>>>>> >
>>>>>> > diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
>>>>>> > index 6df8bd4..cc57911 100644
>>>>>> > --- a/fs/ceph/addr.c
>>>>>> > +++ b/fs/ceph/addr.c
>>>>>> > @@ -746,7 +746,7 @@ retry:
>>>>>> >
>>>>>> > while (!done && index <= end) {
>>>>>> > int num_ops = do_sync ? 2 : 1;
>>>>>> > - unsigned i;
>>>>>> > + unsigned i, j;
>>>>>> > int first;
>>>>>> > pgoff_t next;
>>>>>> > int pvec_pages, locked_pages;
>>>>>> > @@ -894,7 +894,6 @@ get_more_pages:
>>>>>> > if (!locked_pages)
>>>>>> > goto release_pvec_pages;
>>>>>> > if (i) {
>>>>>> > - int j;
>>>>>> > BUG_ON(!locked_pages || first < 0);
>>>>>> >
>>>>>> > if (pvec_pages && i == pvec_pages &&
>>>>>> > @@ -924,7 +923,10 @@ get_more_pages:
>>>>>> >
>>>>>> > osd_req_op_extent_osd_data_pages(req, 0, pages, len, 0,
>>>>>> > !!pool, false);
>>>>>> > -
>>>>>> > + for(j = 0; j < locked_pages; j++) {
>>>>>> > + struct page *page = pages[j];
>>>>>> > + ceph_readpage_to_fscache(inode, page);
>>>>>> > + }
>>>>>> > pages = NULL; /* request message now owns the pages array */
>>>>>> > pool = NULL;
>>>>>> >
>>>>>> > --
>>>>>> > 1.7.9.5
>>>>>> >
>>>>>> > --
>>>>>> > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>>>>>> > the body of a message to majordomo@xxxxxxxxxxxxxxx
>>>>>> > More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Milosz Tanski
>>>>>> CTO
>>>>>> 10 East 53rd Street, 37th floor
>>>>>> New York, NY 10022
>>>>>>
>>>>>> p: 646-253-9055
>>>>>> e: milosz@xxxxxxxxx
>>>>>>
>>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Milosz Tanski
>>>> CTO
>>>> 10 East 53rd Street, 37th floor
>>>> New York, NY 10022
>>>>
>>>> p: 646-253-9055
>>>> e: milosz@xxxxxxxxx
>>>> --
>>>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>>>> the body of a message to majordomo@xxxxxxxxxxxxxxx
>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>> Please read the FAQ at http://www.tux.org/lkml/
>>
>>
>>
>> --
>> Milosz Tanski
>> CTO
>> 10 East 53rd Street, 37th floor
>> New York, NY 10022
>>
>> p: 646-253-9055
>> e: milosz@xxxxxxxxx

--
Milosz Tanski
CTO
10 East 53rd Street, 37th floor
New York, NY 10022

p: 646-253-9055
e: milosz@xxxxxxxxx
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Next message: Axel Lin: "[PATCH] ramfs: nommu: Statize ramfs_nommu_get_unmapped_area andramfs_nommu_mmap"
Previous message: Stephen Rothwell: "linux-next: Tree for Nov 20"
In reply to: Yan, Zheng: "Re: [PATCH] ceph: Update the pages in fscache in writepages() path"
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