Re: [PATCH v1] ext4: fix a assertion failure due to ungranted bh dirting

liubaolin
2024-10-16T13:38:38Z

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> Hello,
> I reviewed the patch attached in your email. The issue you mentioned about clearing buffer_new(bh) in write_end_fn() is indeed a bug.
> However, this patch does not resolve the crash issue we encountered.
>
> Let me explain my analysis in detail below.
> The crash occurs in the function jbd2_journal_dirty_metadata().
>
> ext4_block_write_begin() -> ext4_journalled_zero_new_buffers() -> write_end_fn()
> -> ext4_dirty_journalled_data() -> ext4_handle_dirty_metadata() -> __ext4_handle_dirty_metadata()
> -> jbd2_journal_dirty_metadata()
>
> In the function jbd2_journal_dirty_metadata(), there is the following condition:
> —---------------------------------------------------------------------------------------------------
> if (data_race(jh->b_transaction != transaction &&
> jh->b_next_transaction != transaction)) {
> spin_lock(&jh->b_state_lock);
> J_ASSERT_JH(jh, jh->b_transaction == transaction ||
> jh->b_next_transaction == transaction);
> spin_unlock(&jh->b_state_lock);
> }
> ----------------------------------------------------------------------------------------------------
> By analyzing the vmcore, I found that both jh->b_transaction and jh->b_next_transaction are NULL.
> Through code analysis, I discovered that the __jbd2_journal_file_buffer() function adds the corresponding transaction of bh to jh->b_transaction.
> Normally, this is accessed through do_journal_get_write_access(), which can call __jbd2_journal_file_buffer().
> The detailed function call process is as follows:
> do_journal_get_write_access() -> ext4_journal_get_write_access() -> __ext4_journal_get_write_access()
> -> jbd2_journal_get_write_access() -> do_get_write_access() -> __jbd2_journal_file_buffer()
>
>
> Therefore, resolving the crash issue requires obtaining write access before calling the jbd2_journal_dirty_metadata() function.
> The comment at the definition of the jbd2_journal_dirty_metadata() function also states:
> 'The buffer must have previously had jbd2_journal_get_write_access().'
>
> In the ext4_block_write_begin() function, if get_block() encounters an error, then neither bh->b_this_page nor the subsequent bh calls do_journal_get_write_access().
> If bh->b_this_page and the subsequent bh are in the new state, it will lead to a crash when reaching the jbd2_journal_dirty_metadata() function.
>
> So, there are two ways to resolve this crash issue:
> 1、Call do_journal_get_write_access() on bh that is not handled due to get_block() error.
> The patch modification is in the attachment 0001-ext4-fix-a-assertion-failure-due-to-ungranted-bh-dir.patch.
>
> 2、Call clear_buffer_new() on bh that is not handled due to get_block() error.
> The patch modification is in the attachment 0001-ext4-fix-a-assertion-failure-due-to-bh-not-clear-new.patch.
>
> Additionally, I have found a method to quickly reproduce this crash issue.
> For details, please refer to the email I previously sent you: “https://lore.kernel.org/all/bd41c24b-7325-4584-a965-392a32e32c74@163.com/”.
> I have verified that this quick reproduction method works for both solutions to resolve the issue.
>
> Please continue to consider which method is better to resolve this issue.
> If you think that using clear_buffer_new() is a better solution, I can resend the patch via git send-mail.

在 2024/10/16 18:33, Jan Kara 写道:
> Hello,
>
> On Fri 11-10-24 12:08:58, Baolin Liu wrote:
>> Greetings，
>>
>> This problem is reproduced by our customer using their own testing tool
>> “run_bug”. When I consulted with a client, the testing tool “run_bug”
>> used a variety of background programs to benchmark (including memory
>> pressure, cpu pressure, file cycle manipulation, fsstress Stress testing
>> tool, postmark program，and so on).
>>
>> The recurrence probability is relatively low.
>
> OK, thanks for asking!
>
>> In response to your query, in ext4_block_write_begin, the new state will
>> be clear before get block, and the bh that failed get_block will not be
>> set to new. However, when the page size is greater than the block size, a
>> page will contain multiple bh.
>
> True. I wanted to argue that the buffer_new bit should be either cleared in
> ext4_block_write_begin() (in case of error) or in
> ext4_journalled_write_end() (in case of success) but actually
> ext4_journalled_write_end() misses the clearing. So I think the better
> solution is like the attached patch. I'll submit it once testing finishes
> but it would be great if you could test that it fixes your problems as
> well. Thanks!
>
> Honza

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From cbed522ccb695681d94ec02940e958fcf77e58cd Mon Sep 17 00:00:00 2001
From: Baolin Liu <liubaolin@kylinos.cn>
Date: Wed, 9 Oct 2024 09:30:34 +0800
Subject: [PATCH] ext4: fix a assertion failure due to bh not clear new

Since the merge of commit 3910b513fcdf ("ext4: persist the new uptodate
buffers in ext4_journalled_zero_new_buffers"), a new assertion failure
occurred under a old kernel(ext3, data=journal, pagesize=64k) with
corresponding ported patches:
================================================================
Call trace:
__ext4_handle_dirty_metadata+0x320/0x7e8
write_end_fn+0x78/0x178
ext4_journalled_zero_new_buffers+0xd0/0x2c8
ext4_block_write_begin+0x850/0xc00
ext4_write_begin+0x334/0xc68
generic_perform_write+0x1a4/0x380
ext4_buffered_write_iter+0x180/0x370
ext4_file_write_iter+0x194/0xfc0
new_sync_write+0x338/0x4b8
__vfs_write+0xc4/0xe8
vfs_write+0x12c/0x3d0
ksys_write+0xf4/0x230
sys_write+0x34/0x48
el0_svc_naked+0x44/0x48
================================================================

which was caused by bh dirting without calling clear_buffer_new().

In the loop for all bhs of a page in ext4_block_write_begin(),
when a err occurred, it will jump out of loop.
But that will leaves some bhs being processed and some not,
which will lead to the asserion failure in calling write_end_fn().

To fixed that, clear_buffer_new for the rest unprocessed bhs, just
as what write_end_fn do.

Fixes: 3910b513fcdf ("ext4: persist the new uptodate buffers in ext4_journalled_zero_new_buffers")
Reported-and-tested-by: Zhi Long <longzhi@sangfor.com.cn>
Suggested-by: Shida Zhang <zhangshida@kylinos.cn>
Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
---
fs/ext4/inode.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 54bdd4884fe6..26c107e083c6 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1102,9 +1102,23 @@ int ext4_block_write_begin(handle_t *handle, struct folio *folio,
err = -EIO;
}
if (unlikely(err)) {
- if (should_journal_data)
+ if (should_journal_data) {
+ if (bh != head || !block_start) {
+ do {
+ block_end = block_start + bh->b_size;
+
+ if (buffer_new(bh))
+ if (block_end > from && block_start < to)
+ clear_buffer_new(bh);
+
+ block_start = block_end;
+ bh = bh->b_this_page;
+ } while (bh != head);
+ }
+
ext4_journalled_zero_new_buffers(handle, inode, folio,
from, to);
+ }
else
folio_zero_new_buffers(folio, from, to);
} else if (fscrypt_inode_uses_fs_layer_crypto(inode)) {
--
2.39.2

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From e99bce558ddcc98e2495280e1af0dee372fae619 Mon Sep 17 00:00:00 2001
From: Baolin Liu <liubaolin@kylinos.cn>
Date: Wed, 9 Oct 2024 09:30:34 +0800
Subject: [PATCH v1] ext4: fix a assertion failure due to ungranted bh dirting

Since the merge of commit 3910b513fcdf ("ext4: persist the new uptodate
buffers in ext4_journalled_zero_new_buffers"), a new assertion failure
occurred under a old kernel(ext3, data=journal, pagesize=64k) with
corresponding ported patches:
================================================================
Call trace:
__ext4_handle_dirty_metadata+0x320/0x7e8
write_end_fn+0x78/0x178
ext4_journalled_zero_new_buffers+0xd0/0x2c8
ext4_block_write_begin+0x850/0xc00
ext4_write_begin+0x334/0xc68
generic_perform_write+0x1a4/0x380
ext4_buffered_write_iter+0x180/0x370
ext4_file_write_iter+0x194/0xfc0
new_sync_write+0x338/0x4b8
__vfs_write+0xc4/0xe8
vfs_write+0x12c/0x3d0
ksys_write+0xf4/0x230
sys_write+0x34/0x48
el0_svc_naked+0x44/0x48
================================================================

which was caused by bh dirting without calling
do_journal_get_write_access().

In the loop for all bhs of a page in ext4_block_write_begin(),
when a err occurred, it will jump out of loop.
But that will leaves some bhs being processed and some not,
which will lead to the asserion failure in calling write_end_fn().

To fixed that, get write access for the rest unprocessed bhs, just
as what write_end_fn do.

Fixes: 3910b513fcdf ("ext4: persist the new uptodate buffers in ext4_journalled_zero_new_buffers")
Reported-and-tested-by: Zhi Long <longzhi@sangfor.com.cn>
Suggested-by: Shida Zhang <zhangshida@kylinos.cn>
Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
---
fs/ext4/inode.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 54bdd4884fe6..a72f951288e4 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1102,9 +1102,24 @@ int ext4_block_write_begin(handle_t *handle, struct folio *folio,
err = -EIO;
}
if (unlikely(err)) {
- if (should_journal_data)
+ if (should_journal_data) {
+ if (bh != head || !block_start) {
+ do {
+ block_end = block_start + bh->b_size;
+
+ if (buffer_new(bh))
+ if (block_end > from && block_start < to)
+ do_journal_get_write_access(handle,
+ inode, bh);
+
+ block_start = block_end;
+ bh = bh->b_this_page;
+ } while (bh != head);
+ }
+
ext4_journalled_zero_new_buffers(handle, inode, folio,
from, to);
+ }
else
folio_zero_new_buffers(folio, from, to);
} else if (fscrypt_inode_uses_fs_layer_crypto(inode)) {
--
2.39.2