On Mon, Nov 24, 2008 at 09:03:21PM +0300, Alex Zhuravlev wrote: > Aneesh Kumar K.V wrote: >> On Mon, Nov 24, 2008 at 07:36:49PM +0300, Alex Zhuravlev wrote: >>> Aneesh Kumar K.V wrote: >>>> Ok the changes was not done for this purpose. I need to make sure we >>>> update bitmap and clear group_desc uninit flag after taking sb_bgl_lock >>>> That means when we claim blocks we can't use mb_set_bits with >>>> sb_bgl_lock because we would already be holding it. How about the below >>>> change >>> may I have a look at the original patch? >> >> http://patchwork.ozlabs.org/patch/10065/ > > I don't understand how a group can be "uninit" if we do some manipulations > inside. both allocation and preallocation initialize group first, see in > ext4_mb_init_cache() > With commit c806e68f we do a init_bitmap every time we do a read_block_bitmap. 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 clear EXT4_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_lock and 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 errors EXT4-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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
