Re: [PATCH] jbd2: don't advance j_fc_off before the buffer is recorded

> jbd2_fc_get_buf() increments journal->j_fc_off before jbd2_journal_bmap()
> and __getblk() have had a chance to fail.  When either does, the slot
> j_fc_wbuf[fc_off] is never assigned, but j_fc_off already counts it.
> 
> ext4 then fails the fast commit and falls back, and the fallback path
> reaches jbd2_fc_release_bufs() via ext4_fc_commit ->
> jbd2_fc_end_commit_fallback -> __jbd2_fc_end_commit -> ext4_fc_cleanup.
> That walks down from j_fc_off - 1 and put_bh()es every slot until it
> sees NULL, so it also touches the slot that was never written.
> j_fc_wbuf comes from a plain kmalloc() and is never zeroed, so a slot
> used for the first time holds uninitialised heap data.
> 
> Advance j_fc_off only after the buffer head has been stored.  That
> restores the invariant that j_fc_off covers exactly the filled slots,
> which is what jbd2_fc_release_bufs() relies on: the live buffers of the
> [ ... ]
> 
> Fixes: ff780b91efe9 ("jbd2: add fast commit machinery")
> Reported-by: Yu Junzhe <[email protected]>
> Closes: https://lore.kernel.org/linux-ext4/[email protected]/
> Signed-off-by: Daejun Park <[email protected]>

Sashiko has reviewed this patch and found no issues. It looks great!

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