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Date: Wed, 27 Apr 2016 12:05:56 +0800
From: Ming Lei <ming.lei@...onical.com>
To: NeilBrown <neilb@...e.de>
Cc: Jens Axboe <axboe@...com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
Shaun Tancheff <shaun.tancheff@...gate.com>,
Mikulas Patocka <mpatocka@...hat.com>,
Alan Cox <alan@...ux.intel.com>, Liu Bo <bo.li.liu@...cle.com>,
Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v2 3/3] block: avoid to call .bi_end_io() recursively
On Wed, Apr 27, 2016 at 12:02 PM, NeilBrown <neilb@...e.de> 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 <shaun.tancheff@...gate.com>
>> Cc: Christoph Hellwig <hch@...radead.org>
>> Cc: Mikulas Patocka <mpatocka@...hat.com>
>> Cc: Alan Cox <alan@...ux.intel.com>
>> Cc: Neil Brown <neilb@...e.de>
>> Cc: Liu Bo <bo.li.liu@...cle.com>
>> Signed-off-by: Ming Lei <ming.lei@...onical.com>
>> ---
>> 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
>>
>> --
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