On Tue, Feb 26, 2019 at 07:43:32PM -0700, Jens Axboe wrote:
> On 2/26/19 7:37 PM, Ming Lei wrote:
> > On Tue, Feb 26, 2019 at 07:28:54PM -0700, Jens Axboe wrote:
> >> On 2/26/19 7:21 PM, Ming Lei wrote:
> >>> On Tue, Feb 26, 2019 at 06:57:16PM -0700, Jens Axboe wrote:
> >>>> On 2/26/19 6:53 PM, Ming Lei wrote:
> >>>>> On Tue, Feb 26, 2019 at 06:47:54PM -0700, Jens Axboe wrote:
> >>>>>> On 2/26/19 6:21 PM, Ming Lei wrote:
> >>>>>>> On Tue, Feb 26, 2019 at 11:56 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>> On 2/25/19 9:34 PM, Jens Axboe wrote:
> >>>>>>>>> On 2/25/19 8:46 PM, Eric Biggers wrote:
> >>>>>>>>>> Hi Jens,
> >>>>>>>>>>
> >>>>>>>>>> On Thu, Feb 21, 2019 at 10:45:27AM -0700, Jens Axboe wrote:
> >>>>>>>>>>> On 2/20/19 3:58 PM, Ming Lei wrote:
> >>>>>>>>>>>> On Mon, Feb 11, 2019 at 12:00:41PM -0700, Jens Axboe wrote:
> >>>>>>>>>>>>> For an ITER_BVEC, we can just iterate the iov and add the pages
> >>>>>>>>>>>>> to the bio directly. This requires that the caller doesn't releases
> >>>>>>>>>>>>> the pages on IO completion, we add a BIO_NO_PAGE_REF flag for that.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> The current two callers of bio_iov_iter_get_pages() are updated to
> >>>>>>>>>>>>> check if they need to release pages on completion. This makes them
> >>>>>>>>>>>>> work with bvecs that contain kernel mapped pages already.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>
> >>>>>>>>>>>>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> >>>>>>>>>>>>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> >>>>>>>>>>>>> ---
> >>>>>>>>>>>>> block/bio.c | 59 ++++++++++++++++++++++++++++++++-------
> >>>>>>>>>>>>> fs/block_dev.c | 5 ++--
> >>>>>>>>>>>>> fs/iomap.c | 5 ++--
> >>>>>>>>>>>>> include/linux/blk_types.h | 1 +
> >>>>>>>>>>>>> 4 files changed, 56 insertions(+), 14 deletions(-)
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> diff --git a/block/bio.c b/block/bio.c
> >>>>>>>>>>>>> index 4db1008309ed..330df572cfb8 100644
> >>>>>>>>>>>>> --- a/block/bio.c
> >>>>>>>>>>>>> +++ b/block/bio.c
> >>>>>>>>>>>>> @@ -828,6 +828,23 @@ int bio_add_page(struct bio *bio, struct page *page,
> >>>>>>>>>>>>> }
> >>>>>>>>>>>>> EXPORT_SYMBOL(bio_add_page);
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> +static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
> >>>>>>>>>>>>> +{
> >>>>>>>>>>>>> + const struct bio_vec *bv = iter->bvec;
> >>>>>>>>>>>>> + unsigned int len;
> >>>>>>>>>>>>> + size_t size;
> >>>>>>>>>>>>> +
> >>>>>>>>>>>>> + len = min_t(size_t, bv->bv_len, iter->count);
> >>>>>>>>>>>>> + size = bio_add_page(bio, bv->bv_page, len,
> >>>>>>>>>>>>> + bv->bv_offset + iter->iov_offset);
> >>>>>>>>>>>>
> >>>>>>>>>>>> iter->iov_offset needs to be subtracted from 'len', looks
> >>>>>>>>>>>> the following delta change[1] is required, otherwise memory corruption
> >>>>>>>>>>>> can be observed when running xfstests over loop/dio.
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks, I folded this in.
> >>>>>>>>>>>
> >>>>>>>>>>> --
> >>>>>>>>>>> Jens Axboe
> >>>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> syzkaller started hitting a crash on linux-next starting with this commit, and
> >>>>>>>>>> it still occurs even with your latest version that has Ming's fix folded in.
> >>>>>>>>>> Specifically, commit a566653ab5ab80a from your io_uring branch with commit date
> >>>>>>>>>> Sun Feb 24 08:20:53 2019 -0700.
> >>>>>>>>>>
> >>>>>>>>>> Reproducer:
> >>>>>>>>>>
> >>>>>>>>>> #define _GNU_SOURCE
> >>>>>>>>>> #include <fcntl.h>
> >>>>>>>>>> #include <linux/loop.h>
> >>>>>>>>>> #include <sys/ioctl.h>
> >>>>>>>>>> #include <sys/sendfile.h>
> >>>>>>>>>> #include <sys/syscall.h>
> >>>>>>>>>> #include <unistd.h>
> >>>>>>>>>>
> >>>>>>>>>> int main(void)
> >>>>>>>>>> {
> >>>>>>>>>> int memfd, loopfd;
> >>>>>>>>>>
> >>>>>>>>>> memfd = syscall(__NR_memfd_create, "foo", 0);
> >>>>>>>>>>
> >>>>>>>>>> pwrite(memfd, "\xa8", 1, 4096);
> >>>>>>>>>>
> >>>>>>>>>> loopfd = open("/dev/loop0", O_RDWR|O_DIRECT);
> >>>>>>>>>>
> >>>>>>>>>> ioctl(loopfd, LOOP_SET_FD, memfd);
> >>>>>>>>>>
> >>>>>>>>>> sendfile(loopfd, loopfd, NULL, 1000000);
> >>>>>>>>>> }
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Crash:
> >>>>>>>>>>
> >>>>>>>>>> page:ffffea0001a6aab8 count:0 mapcount:0 mapping:0000000000000000 index:0x0
> >>>>>>>>>> flags: 0x100000000000000()
> >>>>>>>>>> raw: 0100000000000000 ffffea0001ad2c50 ffff88807fca49d0 0000000000000000
> >>>>>>>>>> raw: 0000000000000000 0000000000000000 00000000ffffffff
> >>>>>>>>>> page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
> >>>>>>>>>
> >>>>>>>>> I see what this is, I'll cut a fix for this tomorrow.
> >>>>>>>>
> >>>>>>>> Folded in a fix for this, it's in my current io_uring branch and my for-next
> >>>>>>>> branch.
> >>>>>>>
> >>>>>>> Hi Jens,
> >>>>>>>
> >>>>>>> I saw the following change is added:
> >>>>>>>
> >>>>>>> + if (size == len) {
> >>>>>>> + /*
> >>>>>>> + * For the normal O_DIRECT case, we could skip grabbing this
> >>>>>>> + * reference and then not have to put them again when IO
> >>>>>>> + * completes. But this breaks some in-kernel users, like
> >>>>>>> + * splicing to/from a loop device, where we release the pipe
> >>>>>>> + * pages unconditionally. If we can fix that case, we can
> >>>>>>> + * get rid of the get here and the need to call
> >>>>>>> + * bio_release_pages() at IO completion time.
> >>>>>>> + */
> >>>>>>> + get_page(bv->bv_page);
> >>>>>>>
> >>>>>>> Now the 'bv' may point to more than one page, so the following one may be
> >>>>>>> needed:
> >>>>>>>
> >>>>>>> int i;
> >>>>>>> struct bvec_iter_all iter_all;
> >>>>>>> struct bio_vec *tmp;
> >>>>>>>
> >>>>>>> mp_bvec_for_each_segment(tmp, bv, i, iter_all)
> >>>>>>> get_page(tmp->bv_page);
> >>>>>>
> >>>>>> I guess that would be the safest, even if we don't currently have more
> >>>>>> than one page in there. I'll fix it up.
> >>>>>
> >>>>> It is easy to see multipage bvec from loop, :-)
> >>>>
> >>>> Speaking of this, I took a quick look at why we've now regressed a lot
> >>>> on IOPS perf with the multipage work. It looks like it's all related to
> >>>> the (much) fatter setup around iteration, which is related to this very
> >>>> topic too.
> >>>>
> >>>> Basically setup of things like bio_for_each_bvec() and indexing through
> >>>> nth_page() is MUCH slower than before.
> >>>
> >>> But bio_for_each_bvec() needn't nth_page(), and only bio_for_each_segment()
> >>> needs that. However, bio_for_each_segment() isn't called from
> >>> blk_queue_split() and blk_rq_map_sg().
> >>>
> >>> One issue is that bio_for_each_bvec() still advances by page size
> >>> instead of bvec->len, I guess that is the problem, will cook a patch
> >>> for your test.
> >>
> >> Probably won't make a difference for my test case...
> >>
> >>>> We need to do something about this, it's like tossing out months of
> >>>> optimizations.
> >>>
> >>> Some following optimization can be done, such as removing
> >>> biovec_phys_mergeable() from blk_bio_segment_split().
> >>
> >> I think we really need a fast path for <= PAGE_SIZE IOs, to the extent
> >> that it is possible. But iteration startup cost is a problem in a lot of
> >> spots, and a split fast path will only help a bit for that specific
> >> case.
> >>
> >> 5% regressions is HUGE. I know I've mentioned this before, I just want
> >> to really stress how big of a deal that is. It's enough to make me
> >> consider just reverting it again, which sucks, but I don't feel great
> >> shipping something that is known that much slower.
> >>
> >> Suggestions?
> >
> > You mentioned nth_page() costs much in bio_for_each_bvec(), but which
> > shouldn't call into nth_page(). I will look into it first.
>
> I'll check on the test box tomorrow, I lost connectivity before. I'll
> double check in the morning.
>
> I'd focus on the blk_rq_map_sg() path, since that's the biggest cycle
> consumer.
Hi Jens,
Could you test the following patch which may improve on the 4k randio
test case?
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 066b66430523..c1ad8abbd9d6 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -447,7 +447,7 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
return biovec_phys_mergeable(q, &end_bv, &nxt_bv);
}
-static struct scatterlist *blk_next_sg(struct scatterlist **sg,
+static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
struct scatterlist *sglist)
{
if (!*sg)
@@ -483,7 +483,7 @@ static unsigned blk_bvec_map_sg(struct request_queue *q,
offset = (total + bvec->bv_offset) % PAGE_SIZE;
idx = (total + bvec->bv_offset) / PAGE_SIZE;
- pg = nth_page(bvec->bv_page, idx);
+ pg = bvec_nth_page(bvec->bv_page, idx);
sg_set_page(*sg, pg, seg_size, offset);
@@ -512,7 +512,12 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
(*sg)->length += nbytes;
} else {
new_segment:
- (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
+ if (bvec->bv_offset + bvec->bv_len <= PAGE_SIZE) {
+ *sg = blk_next_sg(sg, sglist);
+ sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
+ (*nsegs) += 1;
+ } else
+ (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
}
*bvprv = *bvec;
}
diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 30a57b68d017..4376f683c08a 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -51,6 +51,11 @@ struct bvec_iter_all {
unsigned done;
};
+static inline struct page *bvec_nth_page(struct page *page, int idx)
+{
+ return idx == 0 ? page : nth_page(page, idx);
+}
+
/*
* various member access, note that bio_data should of course not be used
* on highmem page vectors
@@ -87,8 +92,8 @@ struct bvec_iter_all {
PAGE_SIZE - bvec_iter_offset((bvec), (iter)))
#define bvec_iter_page(bvec, iter) \
- nth_page(mp_bvec_iter_page((bvec), (iter)), \
- mp_bvec_iter_page_idx((bvec), (iter)))
+ bvec_nth_page(mp_bvec_iter_page((bvec), (iter)), \
+ mp_bvec_iter_page_idx((bvec), (iter)))
#define bvec_iter_bvec(bvec, iter) \
((struct bio_vec) { \
@@ -171,7 +176,7 @@ static inline void mp_bvec_last_segment(const struct bio_vec *bvec,
unsigned total = bvec->bv_offset + bvec->bv_len;
unsigned last_page = (total - 1) / PAGE_SIZE;
- seg->bv_page = nth_page(bvec->bv_page, last_page);
+ seg->bv_page = bvec_nth_page(bvec->bv_page, last_page);
/* the whole segment is inside the last page */
if (bvec->bv_offset >= last_page * PAGE_SIZE) {
thanks,
Ming
