[PATCH v2 3/3] block: avoid to call .bi_end_io() recursively

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[PATCH v2 3/3] block: avoid to call .bi_end_io() recursively

Ming Lei
There were reports about heavy stack use by recursive calling
.bi_end_io()([1][2][3]). For example, more than 16K stack is
consumed in a single bio complete path[3], and in [2] stack
overflow can be triggered if 20 nested dm-crypt is used.

Also patches[1] [2] [3] were posted for addressing the issue,
but never be merged. And the idea in these patches is basically
similar, all serializes the recursive calling of .bi_end_io() by
percpu list.

This patch still takes the same idea, but uses bio_list to
implement it, which turns out more simple and the code becomes
more readable meantime.

One corner case which wasn't covered before is that
bi_endio() may be scheduled to run in process context(such
as btrfs), and this patch just bypasses the optimizing for
that case because one new context should have enough stack space,
and this approach isn't capable of optimizing it too because
there isn't easy way to get a per-task linked list head.

xfstests(-g auto) is run with this patch and no regression is
found on ext4, xfs and btrfs.

[1] http://marc.info/?t=121428502000004&r=1&w=2
[2] http://marc.info/?l=dm-devel&m=139595190620008&w=2
[3] http://marc.info/?t=145974644100001&r=1&w=2

Cc: Shaun Tancheff <[hidden email]>
Cc: Christoph Hellwig <[hidden email]>
Cc: Mikulas Patocka <[hidden email]>
Cc: Alan Cox <[hidden email]>
Cc: Neil Brown <[hidden email]>
Cc: Liu Bo <[hidden email]>
Signed-off-by: Ming Lei <[hidden email]>
---
 block/bio.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 55 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 807d25e..6b4ca7b 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -68,6 +68,8 @@ static DEFINE_MUTEX(bio_slab_lock);
 static struct bio_slab *bio_slabs;
 static unsigned int bio_slab_nr, bio_slab_max;
 
+static DEFINE_PER_CPU(struct bio_list *, bio_end_list) = { NULL };
+
 static struct kmem_cache *bio_find_or_create_slab(unsigned int extra_size)
 {
  unsigned int sz = sizeof(struct bio) + extra_size;
@@ -1737,6 +1739,58 @@ static inline bool bio_remaining_done(struct bio *bio)
  return false;
 }
 
+static void __bio_endio(struct bio *bio)
+{
+ if (bio->bi_end_io)
+ bio->bi_end_io(bio);
+}
+
+/* disable local irq when manipulating the percpu bio_list */
+static void unwind_bio_endio(struct bio *bio)
+{
+ struct bio_list *bl;
+ unsigned long flags;
+
+ /*
+ * We can't optimize if bi_endio() is scheduled to run from
+ * process context because there isn't easy way to get a
+ * per-task bio list head or allocate a per-task variable.
+ */
+ if (!in_interrupt()) {
+ /*
+ * It has to be a top calling when it is run from
+ * process context.
+ */
+ WARN_ON(this_cpu_read(bio_end_list));
+ __bio_endio(bio);
+ return;
+ }
+
+ local_irq_save(flags);
+ bl = __this_cpu_read(bio_end_list);
+ if (!bl) {
+ struct bio_list bl_in_stack;
+
+ bl = &bl_in_stack;
+ bio_list_init(bl);
+ __this_cpu_write(bio_end_list, bl);
+ } else {
+ bio_list_add(bl, bio);
+ goto out;
+ }
+
+ while (bio) {
+ local_irq_restore(flags);
+ __bio_endio(bio);
+ local_irq_save(flags);
+
+ bio = bio_list_pop(bl);
+ }
+ __this_cpu_write(bio_end_list, NULL);
+ out:
+ local_irq_restore(flags);
+}
+
 /**
  * bio_endio - end I/O on a bio
  * @bio: bio
@@ -1765,8 +1819,7 @@ again:
  goto again;
  }
 
- if (bio->bi_end_io)
- bio->bi_end_io(bio);
+ unwind_bio_endio(bio);
 }
 EXPORT_SYMBOL(bio_endio);
 
--
1.9.1

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Re: [PATCH v2 3/3] block: avoid to call .bi_end_io() recursively

NeilBrown
On Wed, Apr 27 2016, Ming Lei wrote:

> There were reports about heavy stack use by recursive calling
> .bi_end_io()([1][2][3]). For example, more than 16K stack is
> consumed in a single bio complete path[3], and in [2] stack
> overflow can be triggered if 20 nested dm-crypt is used.
>
> Also patches[1] [2] [3] were posted for addressing the issue,
> but never be merged. And the idea in these patches is basically
> similar, all serializes the recursive calling of .bi_end_io() by
> percpu list.
>
> This patch still takes the same idea, but uses bio_list to
> implement it, which turns out more simple and the code becomes
> more readable meantime.
>
> One corner case which wasn't covered before is that
> bi_endio() may be scheduled to run in process context(such
> as btrfs), and this patch just bypasses the optimizing for
> that case because one new context should have enough stack space,
> and this approach isn't capable of optimizing it too because
> there isn't easy way to get a per-task linked list head.
>
> xfstests(-g auto) is run with this patch and no regression is
> found on ext4, xfs and btrfs.
>
> [1] http://marc.info/?t=121428502000004&r=1&w=2
> [2] http://marc.info/?l=dm-devel&m=139595190620008&w=2
> [3] http://marc.info/?t=145974644100001&r=1&w=2
>
> Cc: Shaun Tancheff <[hidden email]>
> Cc: Christoph Hellwig <[hidden email]>
> Cc: Mikulas Patocka <[hidden email]>
> Cc: Alan Cox <[hidden email]>
> Cc: Neil Brown <[hidden email]>
> Cc: Liu Bo <[hidden email]>
> Signed-off-by: Ming Lei <[hidden email]>
> ---
>  block/bio.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 55 insertions(+), 2 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 807d25e..6b4ca7b 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -68,6 +68,8 @@ static DEFINE_MUTEX(bio_slab_lock);
>  static struct bio_slab *bio_slabs;
>  static unsigned int bio_slab_nr, bio_slab_max;
>  
> +static DEFINE_PER_CPU(struct bio_list *, bio_end_list) = { NULL };
> +
>  static struct kmem_cache *bio_find_or_create_slab(unsigned int extra_size)
>  {
>   unsigned int sz = sizeof(struct bio) + extra_size;
> @@ -1737,6 +1739,58 @@ static inline bool bio_remaining_done(struct bio *bio)
>   return false;
>  }
>  
> +static void __bio_endio(struct bio *bio)
> +{
> + if (bio->bi_end_io)
> + bio->bi_end_io(bio);
> +}
> +
> +/* disable local irq when manipulating the percpu bio_list */
> +static void unwind_bio_endio(struct bio *bio)
> +{
> + struct bio_list *bl;
> + unsigned long flags;
> +
> + /*
> + * We can't optimize if bi_endio() is scheduled to run from
> + * process context because there isn't easy way to get a
> + * per-task bio list head or allocate a per-task variable.
> + */
> + if (!in_interrupt()) {
> + /*
> + * It has to be a top calling when it is run from
> + * process context.
> + */
> + WARN_ON(this_cpu_read(bio_end_list));
> + __bio_endio(bio);
> + return;
> + }
> +
> + local_irq_save(flags);
> + bl = __this_cpu_read(bio_end_list);
> + if (!bl) {
> + struct bio_list bl_in_stack;
> +
> + bl = &bl_in_stack;
> + bio_list_init(bl);
> + __this_cpu_write(bio_end_list, bl);
The patch seems to make sense, but this bit bothers me.
You are expecting bl_in_stack to still be usable after this block of
code completes.  While it probably is, I don't think it is a good idea
to depend on it.
If you move the "struct bio_list bl_in_stack" to the top of the function
I would be a lot happier.

Or you could change the code to:

   if (bl) {
       bio_list_add(bl, bio);
   } else {
       struct bio_list bl_in_stack;
       ... use bl_in_stack,
       while loop
       set bio_end_list to NULL
   }

and the code flow would all be must clearer.

Thanks,
NeilBrown

> + } else {
> + bio_list_add(bl, bio);
> + goto out;
> + }
> +
> + while (bio) {
> + local_irq_restore(flags);
> + __bio_endio(bio);
> + local_irq_save(flags);
> +> + bio = bio_list_pop(bl);
> + }
> + __this_cpu_write(bio_end_list, NULL);
> + out:
> + local_irq_restore(flags);
> +}
> +
>  /**
>   * bio_endio - end I/O on a bio
>   * @bio: bio
> @@ -1765,8 +1819,7 @@ again:
>   goto again;
>   }
>  
> - if (bio->bi_end_io)
> - bio->bi_end_io(bio);
> + unwind_bio_endio(bio);
>  }
>  EXPORT_SYMBOL(bio_endio);
>  
> --
> 1.9.1
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to [hidden email]
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

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Re: [PATCH v2 3/3] block: avoid to call .bi_end_io() recursively

Ming Lei
On Wed, Apr 27, 2016 at 12:02 PM, NeilBrown <[hidden email]> wrote:

> On Wed, Apr 27 2016, Ming Lei wrote:
>
>> There were reports about heavy stack use by recursive calling
>> .bi_end_io()([1][2][3]). For example, more than 16K stack is
>> consumed in a single bio complete path[3], and in [2] stack
>> overflow can be triggered if 20 nested dm-crypt is used.
>>
>> Also patches[1] [2] [3] were posted for addressing the issue,
>> but never be merged. And the idea in these patches is basically
>> similar, all serializes the recursive calling of .bi_end_io() by
>> percpu list.
>>
>> This patch still takes the same idea, but uses bio_list to
>> implement it, which turns out more simple and the code becomes
>> more readable meantime.
>>
>> One corner case which wasn't covered before is that
>> bi_endio() may be scheduled to run in process context(such
>> as btrfs), and this patch just bypasses the optimizing for
>> that case because one new context should have enough stack space,
>> and this approach isn't capable of optimizing it too because
>> there isn't easy way to get a per-task linked list head.
>>
>> xfstests(-g auto) is run with this patch and no regression is
>> found on ext4, xfs and btrfs.
>>
>> [1] http://marc.info/?t=121428502000004&r=1&w=2
>> [2] http://marc.info/?l=dm-devel&m=139595190620008&w=2
>> [3] http://marc.info/?t=145974644100001&r=1&w=2
>>
>> Cc: Shaun Tancheff <[hidden email]>
>> Cc: Christoph Hellwig <[hidden email]>
>> Cc: Mikulas Patocka <[hidden email]>
>> Cc: Alan Cox <[hidden email]>
>> Cc: Neil Brown <[hidden email]>
>> Cc: Liu Bo <[hidden email]>
>> Signed-off-by: Ming Lei <[hidden email]>
>> ---
>>  block/bio.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 55 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/bio.c b/block/bio.c
>> index 807d25e..6b4ca7b 100644
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -68,6 +68,8 @@ static DEFINE_MUTEX(bio_slab_lock);
>>  static struct bio_slab *bio_slabs;
>>  static unsigned int bio_slab_nr, bio_slab_max;
>>
>> +static DEFINE_PER_CPU(struct bio_list *, bio_end_list) = { NULL };
>> +
>>  static struct kmem_cache *bio_find_or_create_slab(unsigned int extra_size)
>>  {
>>       unsigned int sz = sizeof(struct bio) + extra_size;
>> @@ -1737,6 +1739,58 @@ static inline bool bio_remaining_done(struct bio *bio)
>>       return false;
>>  }
>>
>> +static void __bio_endio(struct bio *bio)
>> +{
>> +     if (bio->bi_end_io)
>> +             bio->bi_end_io(bio);
>> +}
>> +
>> +/* disable local irq when manipulating the percpu bio_list */
>> +static void unwind_bio_endio(struct bio *bio)
>> +{
>> +     struct bio_list *bl;
>> +     unsigned long flags;
>> +
>> +     /*
>> +      * We can't optimize if bi_endio() is scheduled to run from
>> +      * process context because there isn't easy way to get a
>> +      * per-task bio list head or allocate a per-task variable.
>> +      */
>> +     if (!in_interrupt()) {
>> +             /*
>> +              * It has to be a top calling when it is run from
>> +              * process context.
>> +              */
>> +             WARN_ON(this_cpu_read(bio_end_list));
>> +             __bio_endio(bio);
>> +             return;
>> +     }
>> +
>> +     local_irq_save(flags);
>> +     bl = __this_cpu_read(bio_end_list);
>> +     if (!bl) {
>> +             struct bio_list bl_in_stack;
>> +
>> +             bl = &bl_in_stack;
>> +             bio_list_init(bl);
>> +             __this_cpu_write(bio_end_list, bl);
>
> The patch seems to make sense, but this bit bothers me.
> You are expecting bl_in_stack to still be usable after this block of
> code completes.  While it probably is, I don't think it is a good idea
> to depend on it.
> If you move the "struct bio_list bl_in_stack" to the top of the function
> I would be a lot happier.
>
> Or you could change the code to:
>
>    if (bl) {
>        bio_list_add(bl, bio);
>    } else {
>        struct bio_list bl_in_stack;
>        ... use bl_in_stack,
>        while loop
>        set bio_end_list to NULL
>    }
>
> and the code flow would all be must clearer.

Yeah, definitely, thanks for the point.

Thanks,

>
> Thanks,
> NeilBrown
>
>> +     } else {
>> +             bio_list_add(bl, bio);
>> +             goto out;
>> +     }
>> +
>> +     while (bio) {
>> +             local_irq_restore(flags);
>> +             __bio_endio(bio);
>> +             local_irq_save(flags);
>> +> +          bio = bio_list_pop(bl);
>> +     }
>> +     __this_cpu_write(bio_end_list, NULL);
>> + out:
>> +     local_irq_restore(flags);
>> +}
>> +
>>  /**
>>   * bio_endio - end I/O on a bio
>>   * @bio:     bio
>> @@ -1765,8 +1819,7 @@ again:
>>               goto again;
>>       }
>>
>> -     if (bio->bi_end_io)
>> -             bio->bi_end_io(bio);
>> +     unwind_bio_endio(bio);
>>  }
>>  EXPORT_SYMBOL(bio_endio);
>>
>> --
>> 1.9.1
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to [hidden email]
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html