Re: [PATCH v2 04/13] md: prepare for managing resync I/O pages in clean way

[Shaohua Li <shli@kernel.org>]

On Thu, Mar 02, 2017 at 10:09:14AM +0800, Ming Lei wrote:
> Hi Shaohua,
> 
> On Wed, Mar 1, 2017 at 7:30 AM, Shaohua Li <shli@kernel.org> wrote:
> > On Tue, Feb 28, 2017 at 11:41:34PM +0800, Ming Lei wrote:
> >> Now resync I/O use bio's bec table to manage pages,
> >> this way is very hacky, and may not work any more
> >> once multipage bvec is introduced.
> >>
> >> So introduce helpers and new data structure for
> >> managing resync I/O pages more cleanly.
> >>
> >> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> >> ---
> >>  drivers/md/md.h | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 54 insertions(+)
> >>
> >> diff --git a/drivers/md/md.h b/drivers/md/md.h
> >> index 1d63239a1be4..b5a638d85cb4 100644
> >> --- a/drivers/md/md.h
> >> +++ b/drivers/md/md.h
> >> @@ -720,4 +720,58 @@ static inline void mddev_check_writesame(struct mddev *mddev, struct bio *bio)
> >>  #define RESYNC_BLOCK_SIZE (64*1024)
> >>  #define RESYNC_PAGES ((RESYNC_BLOCK_SIZE + PAGE_SIZE-1) / PAGE_SIZE)
> >>
> >> +/* for managing resync I/O pages */
> >> +struct resync_pages {
> >> +     unsigned        idx;    /* for get/put page from the pool */
> >> +     void            *raid_bio;
> >> +     struct page     *pages[RESYNC_PAGES];
> >> +};
> >
> > I'd like this to be embedded into r1bio directly. Not sure if we really need a
> > structure.
> 
> There are two reasons we can't put this into r1bio:
> - r1bio is used in both normal and resync I/O, not fair to allocate more
> in normal I/O, and that is why this patch wouldn't like to touch r1bio or r10bio
> 
> - the count of 'struct resync_pages' instance depends on raid_disks(raid1)
> or copies(raid10), both can't be decided during compiling.

Yes, I said it should be a pointer of r1bio pointing to the pages in other emails.
 
> >
> >> +}
> >> +
> >> +static inline bool resync_page_available(struct resync_pages *rp)
> >> +{
> >> +     return rp->idx < RESYNC_PAGES;
> >> +}
> >
> > Then we don't need this obscure API.
> 
> That is fine.
> 
> 
> Thanks,
> Ming Lei
 
> >
> >> +
> >> +static inline int resync_alloc_pages(struct resync_pages *rp,
> >> +                                  gfp_t gfp_flags)
> >> +{
> >> +     int i;
> >> +
> >> +     for (i = 0; i < RESYNC_PAGES; i++) {
> >> +             rp->pages[i] = alloc_page(gfp_flags);
> >> +             if (!rp->pages[i])
> >> +                     goto out_free;
> >> +     }
> >> +
> >> +     return 0;
> >> +
> >> + out_free:
> >> +     while (--i >= 0)
> >> +             __free_page(rp->pages[i]);
> >> +     return -ENOMEM;
> >> +}
> >> +
> >> +static inline void resync_free_pages(struct resync_pages *rp)
> >> +{
> >> +     int i;
> >> +
> >> +     for (i = 0; i < RESYNC_PAGES; i++)
> >> +             __free_page(rp->pages[i]);
> >
> > Since we will use get_page, shouldn't this be put_page?
> 
> You are right, will fix in v3.
> 
> >
> >> +}
> >> +
> >> +static inline void resync_get_all_pages(struct resync_pages *rp)
> >> +{
> >> +     int i;
> >> +
> >> +     for (i = 0; i < RESYNC_PAGES; i++)
> >> +             get_page(rp->pages[i]);
> >> +}
> >> +
> >> +static inline struct page *resync_fetch_page(struct resync_pages *rp)
> >> +{
> >> +     if (WARN_ON_ONCE(rp->idx >= RESYNC_PAGES))
> >> +             return NULL;
> >> +     return rp->pages[rp->idx++];
> >
> > I'd like the caller explicitly specify the index instead of a global variable
> > to track it, which will make the code more understandable and less error prone.
> 
> That is fine, but the index has to be per bio, and finally the index
> has to be stored
> in 'struct resync_pages', so every user has to call it in the following way:
> 
>           resync_fetch_page(rp, rp->idx);
> 
> then looks no benefit to pass index explicitly.

I suppose the callers always iterate though page 0 to RESYNC_PAGES - 1. Then
the callers can use an index by themselves. That will clearly show which page
the callers are using. The resync_fetch_page() wrap is a blackbox we always
need to refer to the definition.

Thanks,
Shaohua