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SubjectRe: [PATCH 15/28] ext4: Calculate and verify block bitmap checksum
On Wed, Oct 12, 2011 at 06:00:40PM -0600, Andreas Dilger wrote:
> On 2011-10-08, at 1:55 AM, Darrick J. Wong wrote:
> > Compute and verify the checksum of the block bitmap; this checksum is
> > stored in the block group descriptor.
> >
> > @@ -353,11 +360,26 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> > /*
> > * file system mounted not to panic on error,
> > + * -EIO with corrupt bitmap
> > */
> > + ext4_lock_group(sb, block_group);
> > + if (!ext4_valid_block_bitmap(sb, desc, block_group, bh) ||
> > + !ext4_block_bitmap_csum_verify(sb, block_group, desc, bh,
> > + EXT4_BLOCKS_PER_GROUP(sb) / 8)) {
> > + ext4_unlock_group(sb, block_group);
> > + put_bh(bh);
> > + ext4_error(sb, "Corrupt block bitmap - block_group = %u, "
> > + "block_bitmap = %llu", block_group, bitmap_blk);
> > + return NULL;
> > + }
> > + ext4_unlock_group(sb, block_group);
> > + set_buffer_verified(bh);
>
> I've been thinking a while that we should add per-group error flags
> for the block and inode bitmaps. That way, if we detect errors with
> either one, we can set the flag in the group descriptor and avoid
> using it for any allocations in the future. Otherwise, we try to
> read the bitmap in repeatedly.

I think there's some code in ext4 somewhere that does that. I also wonder if
the possibility that we're seeing a transient corruption error is worth
rechecking the block until it fails? (I suspect not, but I decided to throw
that out there anyway.)

> > @@ -803,6 +842,11 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
> > if (groups_per_page == 0)
> > groups_per_page = 1;
> >
> > + csd = kzalloc(sizeof(struct ext4_csum_data) * groups_per_page,
> > + GFP_NOFS);
> > + if (csd == NULL)
> > + goto out;
> > +
> > /* allocate buffer_heads to read bitmaps */
> > if (groups_per_page > 1) {
> > err = -ENOMEM;
> > @@ -880,22 +924,25 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
> > * get set with buffer lock held.
> > */
> > set_bitmap_uptodate(bh[i]);
> > - bh[i]->b_end_io = end_buffer_read_sync;
> > + csd[i].cd_sb = sb;
> > + csd[i].cd_group = first_group + i;
> > + bh[i]->b_private = csd + i;
> > + bh[i]->b_end_io = ext4_end_buffer_read_sync;
>
> It seems to be allocating this extra csd[] and calling the more complex
> ext4_end_buffer_read_sync() callback regardless of whether the checksum
> code is enabled or not. Would it be better to only set the custom
> callback if we need to verify the checksum?

Yep, we could go straight to end_buffer_read_sync in the no-csum case.

--D


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