[PATCH] Revert "Revert "block, bfq: honor already-setup queue merges""

[PATCH] Revert "Revert "block, bfq: honor already-setup queue merges""

[Paolo Valente]

A crash [1] happened to be triggered in conjunction with commit
2d52c58b9c9b ("block, bfq: honor already-setup queue merges"). The
latter was then reverted by commit ebc69e897e17 ("Revert "block, bfq:
honor already-setup queue merges""). Yet, the reverted commit was not
the one introducing the bug. In fact, it actually triggered a UAF
introduced by a different commit, and now fixed by commit d29bd41428cf
("block, bfq: reset last_bfqq_created on group change").

So, there is no point in keeping commit 2d52c58b9c9b ("block, bfq:
honor already-setup queue merges") out. This commit restores it.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=214503

Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
 block/bfq-iosched.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index fec18118dc30..7cde7a11c42b 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2662,6 +2662,15 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
 	 * are likely to increase the throughput.
 	 */
 	bfqq->new_bfqq = new_bfqq;
+	/*
+	 * The above assignment schedules the following redirections:
+	 * each time some I/O for bfqq arrives, the process that
+	 * generated that I/O is disassociated from bfqq and
+	 * associated with new_bfqq. Here we increases new_bfqq->ref
+	 * in advance, adding the number of processes that are
+	 * expected to be associated with new_bfqq as they happen to
+	 * issue I/O.
+	 */
 	new_bfqq->ref += process_refs;
 	return new_bfqq;
 }
@@ -2724,6 +2733,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 {
 	struct bfq_queue *in_service_bfqq, *new_bfqq;
 
+	/* if a merge has already been setup, then proceed with that first */
+	if (bfqq->new_bfqq)
+		return bfqq->new_bfqq;
+
 	/*
 	 * Check delayed stable merge for rotational or non-queueing
 	 * devs. For this branch to be executed, bfqq must not be
@@ -2825,9 +2838,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 	if (bfq_too_late_for_merging(bfqq))
 		return NULL;
 
-	if (bfqq->new_bfqq)
-		return bfqq->new_bfqq;
-
 	if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
 		return NULL;
 
-- 
2.20.1

Re: [PATCH] Revert "Revert "block, bfq: honor already-setup queue merges""

[Paolo Valente]

> Il giorno 25 nov 2021, alle ore 19:15, Paolo Valente <paolo.valente@linaro.org> ha scritto:
> 
> A crash [1] happened to be triggered in conjunction with commit
> 2d52c58b9c9b ("block, bfq: honor already-setup queue merges"). The
> latter was then reverted by commit ebc69e897e17 ("Revert "block, bfq:
> honor already-setup queue merges""). Yet, the reverted commit was not
> the one introducing the bug. In fact, it actually triggered a UAF
> introduced by a different commit, and now fixed by commit d29bd41428cf
> ("block, bfq: reset last_bfqq_created on group change").
> 
> So, there is no point in keeping commit 2d52c58b9c9b ("block, bfq:
> honor already-setup queue merges") out. This commit restores it.
> 

Hi,
this patch does not seem to have been applied yet (or at least
commented if there are still problems).

Thanks,
Paolo

> [1] https://bugzilla.kernel.org/show_bug.cgi?id=214503
> 
> Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
> Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
> ---
> block/bfq-iosched.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index fec18118dc30..7cde7a11c42b 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2662,6 +2662,15 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
> 	 * are likely to increase the throughput.
> 	 */
> 	bfqq->new_bfqq = new_bfqq;
> +	/*
> +	 * The above assignment schedules the following redirections:
> +	 * each time some I/O for bfqq arrives, the process that
> +	 * generated that I/O is disassociated from bfqq and
> +	 * associated with new_bfqq. Here we increases new_bfqq->ref
> +	 * in advance, adding the number of processes that are
> +	 * expected to be associated with new_bfqq as they happen to
> +	 * issue I/O.
> +	 */
> 	new_bfqq->ref += process_refs;
> 	return new_bfqq;
> }
> @@ -2724,6 +2733,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> {
> 	struct bfq_queue *in_service_bfqq, *new_bfqq;
> 
> +	/* if a merge has already been setup, then proceed with that first */
> +	if (bfqq->new_bfqq)
> +		return bfqq->new_bfqq;
> +
> 	/*
> 	 * Check delayed stable merge for rotational or non-queueing
> 	 * devs. For this branch to be executed, bfqq must not be
> @@ -2825,9 +2838,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> 	if (bfq_too_late_for_merging(bfqq))
> 		return NULL;
> 
> -	if (bfqq->new_bfqq)
> -		return bfqq->new_bfqq;
> -
> 	if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
> 		return NULL;
> 
> -- 
> 2.20.1
>