Re: [PATCH v2 13/13] md: raid10: avoid direct access to bvec table in handle_reshape_read_error

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On Thu, Mar 02, 2017 at 10:37:49AM +0800, Ming Lei wrote:
> Hi Shaohua,
> 
> On Wed, Mar 1, 2017 at 7:46 AM, Shaohua Li <shli@xxxxxxxxxx> wrote:
> > On Tue, Feb 28, 2017 at 11:41:43PM +0800, Ming Lei wrote:
> >> The cost is 128bytes(8*16) stack space in kernel thread context, and
> >> just use the bio helper to retrieve pages from bio.
> >>
> >> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
> >> ---
> >>  drivers/md/raid10.c | 12 ++++++++++--
> >>  1 file changed, 10 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> >> index 0b97631e3905..6ffb64ab45f8 100644
> >> --- a/drivers/md/raid10.c
> >> +++ b/drivers/md/raid10.c
> >> @@ -4670,7 +4670,15 @@ static int handle_reshape_read_error(struct mddev *mddev,
> >>       struct r10bio *r10b = &on_stack.r10_bio;
> >>       int slot = 0;
> >>       int idx = 0;
> >> -     struct bio_vec *bvec = r10_bio->master_bio->bi_io_vec;
> >> +     struct bio_vec *bvl;
> >> +     struct page *pages[RESYNC_PAGES];
> >> +
> >> +     /*
> >> +      * This bio is allocated in reshape_request(), and size
> >> +      * is still RESYNC_PAGES
> >> +      */
> >> +     bio_for_each_segment_all(bvl, r10_bio->master_bio, idx)
> >> +             pages[idx] = bvl->bv_page;
> >
> > The reshape bio is doing IO against the memory we allocated for r10_bio, I'm
> > wondering why we can't get the pages from r10_bio. In this way, we don't need
> > access the bio_vec any more.
> 
> Reshap read is special and the bio(r10_bio->master_bio) isn't allocated from
> .r10buf_pool, please see reshape_request().

Why does this matter? The bio is still doing IO to the memory allocated in
r10_bio. bi_private->r10_bio->the pages.

My guess why you think the reshape read is special is the bio->bi_private
doesn't pointer to resync_pages. And since you let resync_pages pointer to
r10_bio and not vice versa, we can't find r10_bio, correct? I think this is
another reason why r10_bio embeds resync_pages (I mean a pointer to
resync_pages). With this way I suggested, the reshape read isn't special at
all. If we allocate memory for r10_bio/r1_bio, we shouldn't need access any bio
bvec.

Thanks,
Shaohua



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