On Thu, 8 Mar 2012, Ted Ts'o wrote:
> On Wed, Mar 07, 2012 at 10:27:43PM -0800, Sage Weil wrote:
> >
> > This avoids the problem for devices that don't need stable pages, but
> > doesn't help for those that do (btrfs, raid, iscsi, dif/dix, etc.). It
> > seems to me like a more elegant solution would be to COW the page in the
> > address_space so that you get stable writeback pages without blocking.
> > That's clearly more complex, and I'm sure there are a range of issues
> > involved in making that work, but I would hope that it would be doable
> > with generic MM infrastructure so that everyone would benefit.
>
> Well, even doing a COW (or anything that involves messing with page
> tables) is not free. So even if we can make the cost of stable
> writeback pages cheaper, if we can completely avoid the cost, this
> would be good. I'd also rather fix the performance regression sooner
> rather than later, and I suspect the COW solution is not something
> that could be prepared in time for the upcoming merge window.
Definitely. This patch looks like a fine approach for your situation. I
just don't want the subject to come up without talking about a general
solution. And it's very interesting to hear about a (simple) workload
that is affected by the wait_on_page_writeback().
Thanks-
sage
> Martin, would you be willing to try to get your patch submitted for
> the upcoming merge window? Or I'd be willing to carry your patch and
> then rework Darrick's to use the exported flag, and carry it in my
> tree, maybe that would be better.
>
> > I would love to talk to some MM people at LSF about what it would take to
> > make this work...
>
> Sure, long term, I'm much more sympathetic to iSCSI than I am about
> DIF/DIX (which due to drive manufacturer's pricing strategies I don't
> think it will ever become mainstream --- which was my main concern;
> why should we be inflicting pretty severe performance regressions for
> the common case, just to improve things for obscure high-end hardware?
> It's similar to how Solaris trashed 1 and 2 processor performance,
> because they were optimizing things for their high margin SunFires).
> So getting something which makes page stablization not suck so much in
> the long term seems like a fine goal. But even though we've worked on
> improve SMP scalability in a way that didn't sacrifice 2 and 4-way
> processors, we've still supported compiling with !CONFIG_SMP....
>
> - Ted
>
> >
> > sage
> >
> >
> >
> > >
> > > When submitted I will also send a patch to set .needs_stable_pages in
> > > iscsi when needed.
> > >
> > > Thanks, Martin
> > > Boaz
> > >
> > > > diff --git a/block/blk-settings.c b/block/blk-settings.c
> > > > index 5680b91..442a0df 100644
> > > > --- a/block/blk-settings.c
> > > > +++ b/block/blk-settings.c
> > > > @@ -125,6 +125,7 @@ void blk_set_default_limits(struct queue_limits *lim)
> > > > lim->io_opt = 0;
> > > > lim->misaligned = 0;
> > > > lim->cluster = 1;
> > > > + lim->needs_stable_pages = false;
> > > > }
> > > > EXPORT_SYMBOL(blk_set_default_limits);
> > > >
> > > > @@ -571,6 +572,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
> > > > t->cluster &= b->cluster;
> > > > t->discard_zeroes_data &= b->discard_zeroes_data;
> > > >
> > > > + t->needs_stable_pages &= b->needs_stable_pages;
> > > > +
> > > > /* Physical block size a multiple of the logical block size? */
> > > > if (t->physical_block_size & (t->logical_block_size - 1)) {
> > > > t->physical_block_size = t->logical_block_size;
> > > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > > > index 5b85d91..d464aca 100644
> > > > --- a/block/blk-sysfs.c
> > > > +++ b/block/blk-sysfs.c
> > > > @@ -161,6 +161,11 @@ static ssize_t queue_discard_zeroes_data_show(struct request_queue *q, char *pag
> > > > return queue_var_show(queue_discard_zeroes_data(q), page);
> > > > }
> > > >
> > > > +static ssize_t queue_needs_stable_pages_show(struct request_queue *q, char *page)
> > > > +{
> > > > + return queue_var_show(q->limits.needs_stable_pages, page);
> > > > +}
> > > > +
> > > > static ssize_t queue_write_same_max_show(struct request_queue *q, char *page)
> > > > {
> > > > return sprintf(page, "%llu\n",
> > > > @@ -364,6 +369,11 @@ static struct queue_sysfs_entry queue_discard_zeroes_data_entry = {
> > > > .show = queue_discard_zeroes_data_show,
> > > > };
> > > >
> > > > +static struct queue_sysfs_entry queue_needs_stable_pages_entry = {
> > > > + .attr = {.name = "needs_stable_pages", .mode = S_IRUGO },
> > > > + .show = queue_needs_stable_pages_show,
> > > > +};
> > > > +
> > > > static struct queue_sysfs_entry queue_write_same_max_entry = {
> > > > .attr = {.name = "write_same_max_bytes", .mode = S_IRUGO },
> > > > .show = queue_write_same_max_show,
> > > > @@ -416,6 +426,7 @@ static struct attribute *default_attrs[] = {
> > > > &queue_discard_granularity_entry.attr,
> > > > &queue_discard_max_entry.attr,
> > > > &queue_discard_zeroes_data_entry.attr,
> > > > + &queue_needs_stable_pages_entry.attr,
> > > > &queue_write_same_max_entry.attr,
> > > > &queue_nonrot_entry.attr,
> > > > &queue_nomerges_entry.attr,
> > > > diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> > > > index 26eff46..146bed4 100644
> > > > --- a/drivers/scsi/sd.c
> > > > +++ b/drivers/scsi/sd.c
> > > > @@ -1752,10 +1752,11 @@ static void sd_read_protection_type(struct scsi_disk *sdkp, unsigned char *buffe
> > > > return;
> > > > }
> > > >
> > > > - if (scsi_host_dif_capable(sdp->host, type))
> > > > + if (scsi_host_dif_capable(sdp->host, type)) {
> > > > sd_printk(KERN_NOTICE, sdkp,
> > > > "Enabling DIF Type %u protection\n", type);
> > > > - else
> > > > + sdkp->disk->queue->limits.needs_stable_pages = true;
> > > > + } else
> > > > sd_printk(KERN_NOTICE, sdkp,
> > > > "Disabling DIF Type %u protection\n", type);
> > > > }
> > > > diff --git a/drivers/scsi/sd_dif.c b/drivers/scsi/sd_dif.c
> > > > index 0cb39ff..9dc330c 100644
> > > > --- a/drivers/scsi/sd_dif.c
> > > > +++ b/drivers/scsi/sd_dif.c
> > > > @@ -338,6 +338,8 @@ void sd_dif_config_host(struct scsi_disk *sdkp)
> > > > sd_printk(KERN_NOTICE, sdkp,
> > > > "Enabling DIX %s protection\n", disk->integrity->name);
> > > >
> > > > + disk->queue->limits.needs_stable_pages = true;
> > > > +
> > > > /* Signal to block layer that we support sector tagging */
> > > > if (dif && type && sdkp->ATO) {
> > > > if (type == SD_DIF_TYPE3_PROTECTION)
> > > > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > > > index 92956b7..a5a33db 100644
> > > > --- a/include/linux/blkdev.h
> > > > +++ b/include/linux/blkdev.h
> > > > @@ -266,6 +266,8 @@ struct queue_limits {
> > > > unsigned char discard_misaligned;
> > > > unsigned char cluster;
> > > > unsigned char discard_zeroes_data;
> > > > +
> > > > + bool needs_stable_pages;
> > > > };
> > > >
> > > > struct request_queue {
> > >
> > > --
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> > >
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