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

Shaohua Li
2017-07-12T16:31:12Z

On Wed, Jul 12, 2017 at 09:40:10AM +0800, Ming Lei wrote:
> On Tue, Jul 11, 2017 at 7:14 AM, NeilBrown <neilb@suse.com> wrote:
> > On Mon, Jul 10 2017, Shaohua Li wrote:
> >
> >> On Mon, Jul 10, 2017 at 03:25:41PM +0800, Ming Lei wrote:
> >>> On Mon, Jul 10, 2017 at 02:38:19PM +1000, NeilBrown wrote:
> >>> > On Mon, Jul 10 2017, Ming Lei wrote:
> >>> >
> >>> > > On Mon, Jul 10, 2017 at 11:35:12AM +0800, Ming Lei wrote:
> >>> > >> On Mon, Jul 10, 2017 at 7:09 AM, NeilBrown <neilb@suse.com> wrote:
> >>> > ...
> >>> > >> >> +
> >>> > >> >> + 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?
> >>> > >
> >>> > > Looks I missed the 'return' in mempool_free(), so it is fine.
> >>> > >
> >>> > > How about the following fix?
> >>> >
> >>> > It looks like it would probably work, but it is rather unusual to
> >>> > initialise something just before freeing it.
> >>> >
> >>> > Couldn't you just move the initialization to shortly after the
> >>> > mempool_alloc() call. There looks like a good place that already loops
> >>> > over all the bios....
> >>>
> >>> OK, follows the revised patch according to your suggestion.
> >
> > Thanks.
> >
> > That isn't as tidy as I hoped. So I went deeper into the code to try to
> > understand why...
> >
> > I think that maybe we should just discard the ->idx field completely.
> > It is only used in this code:
> >
> > do {
> > struct page *page;
> > int len = PAGE_SIZE;
> > if (sector_nr + (len>>9) > max_sector)
> > len = (max_sector - sector_nr) << 9;
> > if (len == 0)
> > break;
> > for (bio= biolist ; bio ; bio=bio->bi_next) {
> > struct resync_pages *rp = get_resync_pages(bio);
> > page = resync_fetch_page(rp, rp->idx++);
> > /*
> > * won't fail because the vec table is big enough
> > * to hold all these pages
> > */
> > bio_add_page(bio, page, len, 0);
> > }
> > nr_sectors += len>>9;
> > sector_nr += len>>9;
> > } while (get_resync_pages(biolist)->idx < RESYNC_PAGES);
> >
> > and all of the different 'rp' always have the same value for 'idx'.
> > This code is more complex than it needs to be. This is because it used
> > to be possible for bio_add_page() to fail. That cannot happen any more.
> > So we can make the code something like:
> >
> > for (idx = 0; idx < RESYNC_PAGES; idx++) {
> > struct page *page;
> > int len = PAGE_SIZE;
> > if (sector_nr + (len >> 9) > max_sector)
> > len = (max_sector - sector_nr) << 9
> > if (len == 0)
> > break;
> > for (bio = biolist; bio; bio = bio->bi_next) {
> > struct resync_pages *rp = get_resync_pages(bio);
> > page = resync_fetch_page(rp, idx);
> > bio_add_page(bio, page, len, 0);
> > }
> > nr_sectors += len >> 9;
> > sector_nr += len >> 9;
> > }
> >
> > Or did I miss something?
>
> I think this approach is much clean.

Thought I suggested not using the 'idx' in your previous post, but you said
there is reason (not because of bio_add_page) not to do it. Is that changed?
can't remember the details, I need to dig the mail archives.

Thanks,
Shaohua