Re: [PATCH v3 05/14] md: raid1: don't use bio's vec table to manage resync pages

Ming Lei
2017-07-10T03:35:23Z

On Mon, Jul 10, 2017 at 7:09 AM, NeilBrown <neilb@suse.com> wrote:
> On Fri, Mar 17 2017, Ming Lei wrote:
>
>> Now we allocate one page array for managing resync pages, instead
>> of using bio's vec table to do that, and the old way is very hacky
>> and won't work any more if multipage bvec is enabled.
>>
>> The introduced cost is that we need to allocate (128 + 16) * raid_disks
>> bytes per r1_bio, and it is fine because the inflight r1_bio for
>> resync shouldn't be much, as pointed by Shaohua.
>>
>> Also the bio_reset() in raid1_sync_request() is removed because
>> all bios are freshly new now and not necessary to reset any more.
>>
>> This patch can be thought as a cleanup too
>>
>> Suggested-by: Shaohua Li <shli@kernel.org>
>> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
>> ---
>> drivers/md/raid1.c | 94 +++++++++++++++++++++++++++++++++++++-----------------
>> 1 file changed, 64 insertions(+), 30 deletions(-)
>>
>> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
>> index e30d89690109..0e64beb60e4d 100644
>> --- a/drivers/md/raid1.c
>> +++ b/drivers/md/raid1.c
>> @@ -80,6 +80,24 @@ static void lower_barrier(struct r1conf *conf, sector_t sector_nr);
>> #define raid1_log(md, fmt, args...) \
>> do { if ((md)->queue) blk_add_trace_msg((md)->queue, "raid1 " fmt, ##args); } while (0)
>>
>> +/*
>> + * 'strct resync_pages' stores actual pages used for doing the resync
>> + * IO, and it is per-bio, so make .bi_private points to it.
>> + */
>> +static inline struct resync_pages *get_resync_pages(struct bio *bio)
>> +{
>> + return bio->bi_private;
>> +}
>> +
>> +/*
>> + * for resync bio, r1bio pointer can be retrieved from the per-bio
>> + * 'struct resync_pages'.
>> + */
>> +static inline struct r1bio *get_resync_r1bio(struct bio *bio)
>> +{
>> + return get_resync_pages(bio)->raid_bio;
>> +}
>> +
>> static void * r1bio_pool_alloc(gfp_t gfp_flags, void *data)
>> {
>> struct pool_info *pi = data;
>> @@ -107,12 +125,18 @@ static void * r1buf_pool_alloc(gfp_t gfp_flags, void *data)
>> struct r1bio *r1_bio;
>> struct bio *bio;
>> int need_pages;
>> - int i, j;
>> + int j;
>> + struct resync_pages *rps;
>>
>> r1_bio = r1bio_pool_alloc(gfp_flags, pi);
>> if (!r1_bio)
>> return NULL;
>>
>> + rps = kmalloc(sizeof(struct resync_pages) * pi->raid_disks,
>> + gfp_flags);
>> + if (!rps)
>> + goto out_free_r1bio;
>> +
>> /*
>> * Allocate bios : 1 for reading, n-1 for writing
>> */
>> @@ -132,22 +156,22 @@ static void * r1buf_pool_alloc(gfp_t gfp_flags, void *data)
>> need_pages = pi->raid_disks;
>> else
>> need_pages = 1;
>> - for (j = 0; j < need_pages; j++) {
>> + for (j = 0; j < pi->raid_disks; j++) {
>> + struct resync_pages *rp = &rps[j];
>> +
>> bio = r1_bio->bios[j];
>> - bio->bi_vcnt = RESYNC_PAGES;
>> -
>> - if (bio_alloc_pages(bio, gfp_flags))
>> - goto out_free_pages;
>> - }
>> - /* If not user-requests, copy the page pointers to all bios */
>> - if (!test_bit(MD_RECOVERY_REQUESTED, &pi->mddev->recovery)) {
>> - for (i=0; i<RESYNC_PAGES ; i++)
>> - for (j=1; j<pi->raid_disks; j++) {
>> - struct page *page =
>> - r1_bio->bios[0]->bi_io_vec[i].bv_page;
>> - get_page(page);
>> - r1_bio->bios[j]->bi_io_vec[i].bv_page = page;
>> - }
>> +
>> + if (j < need_pages) {
>> + if (resync_alloc_pages(rp, gfp_flags))
>> + goto out_free_pages;
>> + } else {
>> + memcpy(rp, &rps[0], sizeof(*rp));
>> + resync_get_all_pages(rp);
>> + }
>> +
>> + rp->idx = 0;
>
> This is the only place the ->idx is initialized, in r1buf_pool_alloc().
> The mempool alloc function is suppose to allocate memory, not initialize
> it.
>
> If the mempool_alloc() call cannot allocate memory it will use memory
> from the pool. If this memory has already been used, then it will no
> longer have the initialized value.
>
> In short: you need to initialise memory *after* calling
> mempool_alloc(), unless you ensure it is reset to the init values before
> calling mempool_free().
>
> https://bugzilla.kernel.org/show_bug.cgi?id=196307

OK, thanks for posting it out.

Another fix might be to reinitialize the variable(rp->idx = 0) in
r1buf_pool_free().
Or just set it as zero every time when it is used.

But I don't understand why mempool_free() calls pool->free() at the end of
this function, which may cause to run pool->free() on a new allocated buf,
seems a bug in mempool?

--
Ming Lei