[#LU-7014] IAM index delete operation can require extra credits under certain situations

In testing the nodemap save config patch (http://review.whamcloud.com/#/c/11813/), I ran into a situation where I can consistently cause a kernel panic by creating a large number of index records, and then deleting them. The cause appears to be that under certain circumstances the delete operation requires more credits than are allocated.

Here's the LBUG and jbd2 abort:

...
[ 5846.233511]  [<ffffffffa0c32572>] __ldiskfs_handle_dirty_metadata+0x1d2/0x230 [ldiskfs]
[ 5846.233613]  [<ffffffffa0418592>] ? jbd2_journal_get_write_access+0x32/0x40 [jbd2]
[ 5846.233724]  [<ffffffffa0d15aaa>] iam_txn_dirty+0x2a/0x60 [osd_ldiskfs]
[ 5846.233835]  [<ffffffffa0d18a63>] iam_it_rec_delete+0x4f3/0x6d0 [osd_ldiskfs]
[ 5846.233943]  [<ffffffffa0d191b4>] iam_delete+0x64/0xe0 [osd_ldiskfs]
[ 5846.234073]  [<ffffffffa0d070d7>] osd_index_iam_delete+0x227/0x6c0 [osd_ldiskfs]
[ 5846.234185]  [<ffffffffa0d19df0>] ? iam_lfix_split+0x140/0x140 [osd_ldiskfs]
[ 5846.234347]  [<ffffffffa089f572>] nodemap_idx_delete+0x2e2/0x4e0 [ptlrpc]
...
[ 5846.237738] LDISKFS-fs: iam_txn_dirty:606: aborting transaction: error 28 in __ldiskfs_handle_dirty_metadata
[ 5846.237837] LDISKFS: jbd2_journal_dirty_metadata failed: handle type 0 started at line 991, credits 2/0, errcode -28
[ 5846.237855] LDISKFS-fs error (device loop0) in iam_txn_dirty:608: error 28

[ 5846.243727] Lustre: 9062:0:(osd_iam.c:2410:iam_recycle_leaf()) test-MDT0000: idle blocks failed, will lose the blk 2
[ 5846.243799] Lustre: 9062:0:(osd_internal.h:1087:osd_trans_exec_check()) op 9: used 2, used now 2, reserved 1
[ 5846.243868] Lustre: 9062:0:(osd_handler.c:902:osd_trans_dump_creds())   create: 0/0/0, destroy: 0/0/0
[ 5846.243938] Lustre: 9062:0:(osd_handler.c:909:osd_trans_dump_creds())   attr_set: 0/0/0, xattr_set: 1/1/0
[ 5846.244028] Lustre: 9062:0:(osd_handler.c:919:osd_trans_dump_creds())   write: 0/0/0, punch: 0/0/0, quota 0/0/0
[ 5846.244096] Lustre: 9062:0:(osd_handler.c:926:osd_trans_dump_creds())   insert: 0/0/0, delete: 1/1/2
[ 5846.244165] Lustre: 9062:0:(osd_handler.c:933:osd_trans_dump_creds())   ref_add: 0/0/0, ref_del: 0/0/0
[ 5846.244243] LustreError: 9062:0:(osd_internal.h:1090:osd_trans_exec_check()) LBUG

I also added some debugging code to iam_it_rec_delete to track credits of the form:

CDEBUG(D_INFO, "about to delete, credits: %d\n", h->h_buffer_credits);

Here is the sequence that caused the LBUG:

00000001:00000040:1.0:1439827725.734445:0:9062:0:(osd_iam.c:2443:iam_it_rec_delete()) about to delete, credits: 2
00000001:00000040:1.0:1439827725.734447:0:9062:0:(osd_iam.c:2445:iam_it_rec_delete()) about to dirty, credits: 2
00000001:00000040:1.0:1439827725.734449:0:9062:0:(osd_iam.c:2451:iam_it_rec_delete()) about to shrink, credits: 1                                                                                                              
00000001:00000040:1.0:1439827725.734453:0:9062:0:(osd_iam.c:2460:iam_it_rec_delete()) about to recycle, credits: 0

It looks like dirty, shrink , and recycle can each consume a credit, but don't always:

00000001:00000040:1.0:1439827725.620483:0:9002:0:(osd_iam.c:2443:iam_it_rec_delete()) about to delete, credits: 2
00000001:00000040:1.0:1439827725.620485:0:9002:0:(osd_iam.c:2445:iam_it_rec_delete()) about to dirty, credits: 2
00000001:00000040:1.0:1439827725.620486:0:9002:0:(osd_iam.c:2451:iam_it_rec_delete()) about to shrink, credits: 2
00000001:00000040:1.0:1439827725.620490:0:9002:0:(osd_iam.c:2460:iam_it_rec_delete()) about to recycle, credits: 2

It seems like you could just modify osd_dto_credits_noquota[DTO_INDEX_DELETE] to be 2 instead of 1, but I wasn't sure what the consequence of that would be. I also noticed that in osd_declare_object_destroy(), there are extra credits taken for the recycle operation:

        /* Recycle idle OI leaf may cause additional three OI blocks                                                  
         * to be changed. */                                                                                          
        osd_trans_declare_op(env, oh, OSD_OT_DELETE,                                                                  
                             osd_dto_credits_noquota[DTO_INDEX_DELETE] + 3);

Anyway, I wasn't sure what the best way to proceed is and was hoping for some insight. Thanks.

[LU-7014] IAM index delete operation can require extra credits under certain situations Created: 17/Aug/15 Updated: 19/Sep/15 Resolved: 19/Sep/15
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	None
Fix Version/s:	Lustre 2.8.0

[LU-7014] IAM index delete operation can require extra credits under certain situations Created: 17/Aug/15 Updated: 19/Sep/15 Resolved: 19/Sep/15