Re: [PATCH V4 00/13] MD: a caching layer for raid5/6

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

Hi Shaohua,

I gave it a quick test(dd if=/dev/zero of=/dev/md0) in a KVM, and
here I met a kernel oops.

    [   14.768563] BUG: unable to handle kernel NULL pointer dereference at   (null)
    [   14.769153] IP: [<c1383b13>] xor_sse_3_pf64+0x67/0x207
    [   14.769585] *pde = 00000000
    [   14.769822] Oops: 0000 [#1] SMP
    [   14.770092] Modules linked in:
    [   14.770349] CPU: 0 PID: 2234 Comm: md0_raid5 Not tainted 4.1.0+ #18
    [   14.770854] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
    [   14.771859] task: f62c5e80 ti: f04ec000 task.ti: f04ec000
    [   14.772287] EIP: 0060:[<c1383b13>] EFLAGS: 00010246 CPU: 0
    [   14.772327] EIP is at xor_sse_3_pf64+0x67/0x207
    [   14.772327] EAX: 00000010 EBX: 00000000 ECX: e2a99000 EDX: f48ae000
    [   14.772327] ESI: f48ae000 EDI: 00000010 EBP: f04edcd8 ESP: f04edccc
    [   14.772327]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
    [   14.772327] CR0: 80050033 CR2: 00000000 CR3: 22ac5000 CR4: 000006d0
    [   14.772327] Stack:
    [   14.772327]  c1c4d96c 00000000 00000001 f04edcf8 c1382bcf 00000000 00000002 c112d2e5
    [   14.772327]  00000000 f04edd98 00000002 f04edd30 c138554b f6a2e028 00000001 f6f51b30
    [   14.772327]  00000002 00000000 f48ae000 f6a2e028 00001000 f04edd40 00000002 00000000
    [   14.772327] Call Trace:
    [   14.772327]  [<c1382bcf>] xor_blocks+0x3a/0x6d
    [   14.772327]  [<c112d2e5>] ? page_address+0xb8/0xc0
    [   14.772327]  [<c138554b>] async_xor+0xf3/0x113
    [   14.772327]  [<c186d954>] raid_run_ops+0xf73/0x1025
    [   14.772327]  [<c186d954>] ? raid_run_ops+0xf73/0x1025
    [   14.772327]  [<c16ce8b9>] handle_stripe+0x120/0x199b
    [   14.772327]  [<c10c7df4>] ? clockevents_program_event+0xfb/0x118
    [   14.772327]  [<c10c94f7>] ? tick_program_event+0x5f/0x68
    [   14.772327]  [<c10be529>] ? hrtimer_interrupt+0xa3/0x134
    [   14.772327]  [<c16d0366>] handle_active_stripes.isra.37+0x232/0x2b5
    [   14.772327]  [<c16d0701>] raid5d+0x267/0x44c
    [   14.772327]  [<c18717e6>] ? schedule_timeout+0x1c/0x14d
    [   14.772327]  [<c1872011>] ? _raw_spin_lock_irqsave+0x1f/0x3d
    [   14.772327]  [<c16f684a>] md_thread+0x103/0x112
    [   14.772327]  [<c10a3146>] ? wait_woken+0x62/0x62
    [   14.772327]  [<c16f6747>] ? md_wait_for_blocked_rdev+0xda/0xda
    [   14.772327]  [<c108d44e>] kthread+0xa4/0xa9
    [   14.772327]  [<c1872401>] ret_from_kernel_thread+0x21/0x30
    [   14.772327]  [<c108d3aa>] ? kthread_create_on_node+0x104/0x104
    

I dug it a while, and came up with following fix. Does it make sense
to you?

----
diff --git a/crypto/async_tx/async_xor.c b/crypto/async_tx/async_xor.c
index e1bce26..063ac50 100644
--- a/crypto/async_tx/async_xor.c
+++ b/crypto/async_tx/async_xor.c
@@ -30,6 +30,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/raid/xor.h>
 #include <linux/async_tx.h>
+#include <linux/highmem.h>

 /* do_async_xor - dma map the pages and perform the xor with an engine */
 static __async_inline struct dma_async_tx_descriptor *
@@ -127,7 +128,7 @@ do_sync_xor(struct page *dest, struct page **src_list, unsigned int offset,
        /* convert to buffer pointers */
        for (i = 0; i < src_cnt; i++)
                if (src_list[i])
-                       srcs[xor_src_cnt++] = page_address(src_list[i]) + offset;
+                       srcs[xor_src_cnt++] = kmap_atomic(src_list[i]) + offset;
        src_cnt = xor_src_cnt;
        /* set destination address */
        dest_buf = page_address(dest) + offset;
@@ -140,6 +141,9 @@ do_sync_xor(struct page *dest, struct page **src_list, unsigned int offset,
                xor_src_cnt = min(src_cnt, MAX_XOR_BLOCKS);
                xor_blocks(xor_src_cnt, len, dest_buf, &srcs[src_off]);

+               for(i = src_off; i < src_off + xor_src_cnt; i++)
+                       kunmap_atomic(srcs[i]);
+
                /* drop completed sources */
                src_cnt -= xor_src_cnt;
                src_off += xor_src_cnt;



Thanks.

	--yliu

On Tue, Jun 23, 2015 at 02:37:50PM -0700, Shaohua Li wrote:
> Hi,
> 
> This is the V4 version of the raid5/6 caching layer patches. The patches add
> a caching layer for raid5/6. The caching layer uses a SSD as a cache for a raid
> 5/6. It works like the similar way of a hardware raid controller. The purpose
> is to improve raid performance (reduce read-modify-write) and fix write hole
> issue.
> 
> I split the patch to into smaller ones and hopefuly they are easier to
> understand. The splitted patches will not break bisect. Functions of main parts
> are divided well, though some data structures not.
> 
> I detached the multiple reclaim thread patch, the patch set is aimed to make
> basic logic ready, and we can improve performance later (as long as we can make
> sure current logic is flexible to have improvement space for performance).
> 
> Neil,
> 
> For the issue if flush_start block is required, I double checked it. You are
> right we don't really need it, the data/parity checksum stored in cache disk
> will guarantee data integrity, but we can't use a simple disk cache flush as
> there is ordering issue. So the flush_start block does increase overhead, but
> might not too much. I didn't delete it yet, but I'm open to do it.
> 
> Thanks,
> Shaohua
> 
> V4:
> -split patche into smaller ones
> -add more comments into code and some code cleanup
> -bug fixes in recovery code
> -fix the feature bit
> 
> V3:
> -make reclaim multi-thread
> -add statistics in sysfs
> -bug fixes
> 
> V2:
> -metadata write doesn't use FUA
> -discard request is only issued when necessary
> -bug fixes and cleanup
> 
> Shaohua Li (12):
>   raid5: directly use mddev->queue
>   raid5: cache log handling
>   raid5: cache part of raid5 cache
>   raid5: cache reclaim support
>   raid5: cache IO error handling
>   raid5: cache device quiesce support
>   raid5: cache recovery support
>   raid5: add some sysfs entries
>   raid5: don't allow resize/reshape with cache support
>   raid5: guarantee cache release stripes in correct way
>   raid5: enable cache for raid array with cache disk
>   raid5: skip resync if caching is enabled
> 
> Song Liu (1):
>   MD: add a new disk role to present cache device
> 
>  drivers/md/Makefile            |    2 +-
>  drivers/md/md.c                |   24 +-
>  drivers/md/md.h                |    4 +
>  drivers/md/raid5-cache.c       | 3755 ++++++++++++++++++++++++++++++++++++++++
>  drivers/md/raid5.c             |  176 +-
>  drivers/md/raid5.h             |   24 +
>  include/uapi/linux/raid/md_p.h |   79 +
>  7 files changed, 4016 insertions(+), 48 deletions(-)
>  create mode 100644 drivers/md/raid5-cache.c
> 
> -- 
> 1.8.1
> 
> --
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