On Mon, Dec 17, 2012 at 11:23:59AM +0100, Jan Kara wrote: > On Thu 13-12-12 00:08:11, Darrick J. Wong wrote: > > Several complaints have been received regarding long file write latencies when > > memory pages must be held stable during writeback. Since it might not be > > acceptable to stall programs for the entire duration of a page write (which may > > take many milliseconds even on good hardware), enable a second strategy wherein > > pages are snapshotted as part of submit_bio; the snapshot can be held stable > > while writes continue. > > > > This provides a band-aid to provide stable page writes on jbd without needing > > to backport the fixed locking scheme in jbd2. A mount option is added to ext4 > > to allow administrators to enable it there. > > > > For those wondering about the ext3 bandage -- fixing the jbd locking (which was > > done as part of ext4dev years ago) is a lot of surgery, and setting > > PG_writeback on data pages when we actually hold the page lock dropped ext3 > > performance by nearly an order of magnitude. If we're going to migrate iscsi > > and raid to use stable page writes, the complaints about high latency will > > likely return. We might as well centralize their page snapshotting thing to > > one place. > Umm, I kind of like this solution for ext3... :) > > diff --git a/mm/Kconfig b/mm/Kconfig > > index a3f8ddd..78db0e1 100644 > > --- a/mm/Kconfig > > +++ b/mm/Kconfig > > @@ -224,6 +224,16 @@ config BOUNCE > > def_bool y > > depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM) > > > > +# On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often > > +# have more than 4GB of memory, but we don't currently use the IOTLB to present > > +# a 32-bit address to OHCI. So we need to use a bounce pool instead. > > +# > > +# We also use the bounce pool to provide stable page writes for users that > > +# don't (or can't) afford the wait latency. > > +config NEED_BOUNCE_POOL > > + bool > > + default y if (TILE && USB_OHCI_HCD) || (BLK_DEV_INTEGRITY && (EXT3_FS || EXT4_FS)) > > + > This means that NEED_BOUNCE_POOL is going to be enabled on pretty much > any distro kernel... I want to drop EXT4_FS from that default line. Then we only need it in the case where EXT4_USE_FOR_EXT23=y hasn't taken the place of EXT3_FS=y. <shrug> Has anyone actually done that? Heh, even *I* haven't done that. > > config NR_QUICK > > int > > depends on QUICKLIST > > diff --git a/mm/bounce.c b/mm/bounce.c > > index 0420867..fa11935 100644 > > --- a/mm/bounce.c > > +++ b/mm/bounce.c > > @@ -178,6 +178,38 @@ static void bounce_end_io_read_isa(struct bio *bio, int err) > > __bounce_end_io_read(bio, isa_page_pool, err); > > } > > > > +#ifdef CONFIG_NEED_BOUNCE_POOL > > +static int might_snapshot_stable_page_write(struct bio **bio_orig) > > +{ > > + return bio_data_dir(*bio_orig) == WRITE; > > +} > ... so might might_snapshot_stable_page_write() will be true for each > write. And thus blk_queue_bounce() becomes considerably more expensive? > Also calling should_snapshot_stable_pages() for every page seems to be > stupid since its result is going to be the same for all the pages in the > bio (well, I could imagine setups where it won't be but do we want to > support them?). > > So cannot we just make a function like should_snapshot_stable_pages() to > test whether we really need the bouncing, use it in blk_queue_bounce() and > then pass the information to __blk_queue_bounce() if needed? Yes. I'd actually considered simply calling should_snapshot() on the first bio_vec page and passing that information through, but thought I should play it safe for the first revision. However, a bio targets a single block device, so this seems like a safe optimization. --D > > Honza > > > +static int should_snapshot_stable_pages(struct page *page, int rw) > > +{ > > + struct backing_dev_info *bdi; > > + struct address_space *mapping = page_mapping(page); > > + > > + if (!mapping) > > + return 0; > > + bdi = mapping->backing_dev_info; > > + if (!bdi_cap_stable_pages_required(bdi)) > > + return 0; > > + > > + return mapping->host->i_sb->s_flags & MS_SNAP_STABLE && > > + rw == WRITE; > > +} > > +#else > > +static int might_snapshot_stable_page_write(struct bio **bio_orig) > > +{ > > + return 0; > > +} > > + > > +static int should_snapshot_static_pages(struct page *page, int rw) > > +{ > > + return 0; > > +} > > +#endif /* CONFIG_NEED_BOUNCE_POOL */ > > + > > static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig, > > mempool_t *pool) > > { > > @@ -192,7 +224,8 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig, > > /* > > * is destination page below bounce pfn? > > */ > > - if (page_to_pfn(page) <= queue_bounce_pfn(q)) > > + if (page_to_pfn(page) <= queue_bounce_pfn(q) && > > + !should_snapshot_stable_pages(page, rw)) > > continue; > > > > /* > > @@ -284,7 +317,8 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig) > > * don't waste time iterating over bio segments > > */ > > if (!(q->bounce_gfp & GFP_DMA)) { > > - if (queue_bounce_pfn(q) >= blk_max_pfn) > > + if (queue_bounce_pfn(q) >= blk_max_pfn && > > + !might_snapshot_stable_page_write(bio_orig)) > > return; > > pool = page_pool; > > } else { > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > > index 3e4a8cc..fbd8efb 100644 > > --- a/mm/page-writeback.c > > +++ b/mm/page-writeback.c > > @@ -2291,6 +2291,10 @@ void wait_for_stable_page(struct page *page) > > > > if (!bdi_cap_stable_pages_required(bdi)) > > return; > > +#ifdef CONFIG_NEED_BOUNCE_POOL > > + if (mapping->host->i_sb->s_flags & MS_SNAP_STABLE) > > + return; > > +#endif /* CONFIG_NEED_BOUNCE_POOL */ > > > > wait_on_page_writeback(page); > > } > > > -- > Jan Kara <jack@xxxxxxx> > SUSE Labs, CR > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html