Re: [RFC 1/6] ext4: Fixes ext4_mb_mark_bb() with flex_bg with fast_commit

[Ritesh Harjani <riteshh@linux.ibm.com>]

On 22/02/01 12:21PM, Jan Kara wrote:
> 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@linux.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) {
> 		...
> 	}

Sure, will check.

>
> > +	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.

Agree, will make this change.

-ritesh