Aneesh Kumar K.V wrote:
With commit c806e68f we do a init_bitmap every time we do a read_block_bitmap.
can you explain why do we need to init it every time? thanks, Alex
To quote the update commit message that i have ext4: Fix race between read_block_bitmap() and mark_diskspace_used()We need to make sure we update the block bitmap and clearEXT4_BG_BLOCK_UNINIT flag with sb_bgl_lock held. We look at EXT4_BG_BLOCK_UNINIT and reinit the block bitmap each time in ext4_read_block_bitmap (introduced by commit c806e68f), and this can race with block allocations in ext4_mb_mark_diskspace_used().ext4_read_block_bitmap does: spin_lock(sb_bgl_lock(EXT4_SB(sb), block_group));if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) { ext4_init_block_bitmap(sb, bh, block_group, desc);Now on the block allocation side we do mb_set_bits(sb_bgl_lock(sbi, ac->ac_b_ex.fe_group), bitmap_bh->b_data,ac->ac_b_ex.fe_start, ac->ac_b_ex.fe_len); .... spin_lock(sb_bgl_lock(sbi, ac->ac_b_ex.fe_group)); if (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) { gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);ie on allocation we update the bitmap then we take the sb_bgl_lockand clear the EXT4_BG_BLOCK_UNINIT flag. What can happen is a parallel ext4_read_block_bitmap can zero out the bitmap in between the above mb_set_bits and spin_lock(sb_bg_lock..)The race results in below user visible errorsEXT4-fs error (device sdb1): ext4_mb_release_inode_pa: free 100, pa_free 105 EXT4-fs error (device sdb1): mb_free_blocks: double-free of inode 0's block 50(bit 100 in group 0) -aneesh
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