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Message-ID: <20220201112134.aps3kd2ffv4trlhs@quack3.lan>
Date: Tue, 1 Feb 2022 12:21:34 +0100
From: Jan Kara <jack@...e.cz>
To: Ritesh Harjani <riteshh@...ux.ibm.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: Re: [RFC 1/6] ext4: Fixes ext4_mb_mark_bb() with flex_bg with
fast_commit
On Mon 31-01-22 20:46:50, Ritesh Harjani wrote:
> In case of flex_bg feature (which is by default enabled), extents for
> any given inode might span across blocks from two different block group.
> ext4_mb_mark_bb() only reads the buffer_head of block bitmap once for the
> starting block group, but it fails to read it again when the extent length
> boundary overflows to another block group. Then in this below loop it
> accesses memory beyond the block group bitmap buffer_head and results
> into a data abort.
>
> for (i = 0; i < clen; i++)
> if (!mb_test_bit(blkoff + i, bitmap_bh->b_data) == !state)
> already++;
>
> This patch adds this functionality for checking block group boundary in
> ext4_mb_mark_bb() and update the buffer_head(bitmap_bh) for every different
> block group.
>
> w/o this patch, I was easily able to hit a data access abort using Power platform.
>
> <...>
> [ 74.327662] EXT4-fs error (device loop3): ext4_mb_generate_buddy:1141: group 11, block bitmap and bg descriptor inconsistent: 21248 vs 23294 free clusters
> [ 74.533214] EXT4-fs (loop3): shut down requested (2)
> [ 74.536705] Aborting journal on device loop3-8.
> [ 74.702705] BUG: Unable to handle kernel data access on read at 0xc00000005e980000
> [ 74.703727] Faulting instruction address: 0xc0000000007bffb8
> cpu 0xd: Vector: 300 (Data Access) at [c000000015db7060]
> pc: c0000000007bffb8: ext4_mb_mark_bb+0x198/0x5a0
> lr: c0000000007bfeec: ext4_mb_mark_bb+0xcc/0x5a0
> sp: c000000015db7300
> msr: 800000000280b033
> dar: c00000005e980000
> dsisr: 40000000
> current = 0xc000000027af6880
> paca = 0xc00000003ffd5200 irqmask: 0x03 irq_happened: 0x01
> pid = 5167, comm = mount
> <...>
> enter ? for help
> [c000000015db7380] c000000000782708 ext4_ext_clear_bb+0x378/0x410
> [c000000015db7400] c000000000813f14 ext4_fc_replay+0x1794/0x2000
> [c000000015db7580] c000000000833f7c do_one_pass+0xe9c/0x12a0
> [c000000015db7710] c000000000834504 jbd2_journal_recover+0x184/0x2d0
> [c000000015db77c0] c000000000841398 jbd2_journal_load+0x188/0x4a0
> [c000000015db7880] c000000000804de8 ext4_fill_super+0x2638/0x3e10
> [c000000015db7a40] c0000000005f8404 get_tree_bdev+0x2b4/0x350
> [c000000015db7ae0] c0000000007ef058 ext4_get_tree+0x28/0x40
> [c000000015db7b00] c0000000005f6344 vfs_get_tree+0x44/0x100
> [c000000015db7b70] c00000000063c408 path_mount+0xdd8/0xe70
> [c000000015db7c40] c00000000063c8f0 sys_mount+0x450/0x550
> [c000000015db7d50] c000000000035770 system_call_exception+0x4a0/0x4e0
> [c000000015db7e10] c00000000000c74c system_call_common+0xec/0x250
> --- Exception: c00 (System Call) at 00007ffff7dbfaa4
>
> Fixes: 8016e29f4362e28 ("ext4: fast commit recovery path")
> Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> ---
> fs/ext4/mballoc.c | 30 +++++++++++++++++++++++++++---
> 1 file changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index c781974df9d0..8d23108cf9d7 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3899,12 +3899,29 @@ void ext4_mb_mark_bb(struct super_block *sb, ext4_fsblk_t block,
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> ext4_group_t group;
> ext4_grpblk_t blkoff;
> - int i, clen, err;
> + int i, err;
> int already;
> + unsigned int clen, overflow;
>
> - clen = EXT4_B2C(sbi, len);
> -
> +again:
And maybe structure this as a while loop? Like:
while (len > 0) {
...
}
> + overflow = 0;
> ext4_get_group_no_and_offset(sb, block, &group, &blkoff);
> +
> + /*
> + * Check to see if we are freeing blocks across a group
> + * boundary.
> + * In case of flex_bg, this can happen that (block, len) may span across
> + * more than one group. In that case we need to get the corresponding
> + * group metadata to work with. For this we have goto again loop.
> + */
> + if (EXT4_C2B(sbi, blkoff) + len > EXT4_BLOCKS_PER_GROUP(sb)) {
> + overflow = EXT4_C2B(sbi, blkoff) + len -
> + EXT4_BLOCKS_PER_GROUP(sb);
> + len -= overflow;
Why not just:
thisgrp_len = min_t(int, len,
EXT4_BLOCKS_PER_GROUP(sb) - EXT4_C2B(sbi, blkoff));
clen = EXT4_NUM_B2C(sbi, thisgrp_len);
It seems easier to understand to me.
Honza
> + }
> +
> + clen = EXT4_NUM_B2C(sbi, len);
> +
> bitmap_bh = ext4_read_block_bitmap(sb, group);
> if (IS_ERR(bitmap_bh)) {
> err = PTR_ERR(bitmap_bh);
> @@ -3960,6 +3977,13 @@ void ext4_mb_mark_bb(struct super_block *sb, ext4_fsblk_t block,
> err = ext4_handle_dirty_metadata(NULL, NULL, gdp_bh);
> sync_dirty_buffer(gdp_bh);
>
> + if (overflow && !err) {
> + block += len;
> + len = overflow;
> + put_bh(bitmap_bh);
> + goto again;
> + }
> +
> out_err:
> brelse(bitmap_bh);
> }
> --
> 2.31.1
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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