Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV_INTEGRITY

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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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