On Thu, 9 Jun 2011 03:04:33 +0900 Namhyung Kim <namhyung@xxxxxxxxx> wrote:
> When performing a recovery, only first 2 slots in r10_bio are in use,
> for read and write respectively. However all of pages in the write bio
> are never used and just replaced to read bio's when the read completes.
>
> Get rid of those unused pages and share read pages properly.
>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxx>
> ---
> drivers/md/raid10.c | 13 ++++++++++---
> 1 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index a53779ffdf89..621594981339 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -116,6 +116,13 @@ static void * r10buf_pool_alloc(gfp_t gfp_flags, void *data)
> goto out_free_bio;
> r10_bio->devs[j].bio = bio;
> }
> +
> + /*
> + * We can share bv_page's during the recovery
> + */
> + if (!test_bit(MD_RECOVERY_SYNC, &conf->mddev->recovery))
> + nalloc--;
> +
> /*
> * Allocate RESYNC_PAGES data pages and attach them
> * where needed.
> @@ -1363,16 +1370,16 @@ static void recovery_request_write(mddev_t *mddev, r10bio_t *r10_bio)
> int i, d;
> struct bio *bio, *wbio;
>
> -
> - /* move the pages across to the second bio
> + /*
> + * share the pages with the first bio
> * and submit the write request
> */
> bio = r10_bio->devs[0].bio;
> wbio = r10_bio->devs[1].bio;
> for (i=0; i < wbio->bi_vcnt; i++) {
> struct page *p = bio->bi_io_vec[i].bv_page;
> - bio->bi_io_vec[i].bv_page = wbio->bi_io_vec[i].bv_page;
> wbio->bi_io_vec[i].bv_page = p;
> + get_page(p);
> }
> d = r10_bio->devs[1].devnum;
>
Thanks. Interesting idea, but I don't think this code is safe.
We end up calling bio_add_page on with an uninitialised 'page' (in
sync_request()). This could result on BIOVEC_PHYS_MERGEABLE doing funny
things.
It would be OK to set up the two links to the one page in r10buf_pool_alloc,
so recovery_request_write doesn't need to do anything with pages.
That would probably be even more safe than the current code.
Thanks,
NeilBrown
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