Re: [PATCH 10/14] bdi: remove BDI_CAP_SYNCHRONOUS_IO

From: Minchan Kim
Date: Sun Jul 26 2020 - 15:06:47 EST

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In reply to: Christoph Hellwig: "[PATCH 10/14] bdi: remove BDI_CAP_SYNCHRONOUS_IO"
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On Sun, Jul 26, 2020 at 05:03:29PM +0200, Christoph Hellwig wrote:
> BDI_CAP_SYNCHRONOUS_IO is only checked in the swap code, and used to
> decided if ->rw_page can be used on a block device. Just check up for
> the method instead. The only complication is that zram needs a second
> set of block_device_operations as it can switch between modes that
> actually support ->rw_page and those who don't.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> drivers/block/brd.c | 1 -
> drivers/block/zram/zram_drv.c | 19 +++++++++++++------
> drivers/nvdimm/btt.c | 2 --
> drivers/nvdimm/pmem.c | 1 -
> include/linux/backing-dev.h | 9 ---------
> mm/swapfile.c | 2 +-
> 6 files changed, 14 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/block/brd.c b/drivers/block/brd.c
> index 2723a70eb85593..cc49a921339f77 100644
> --- a/drivers/block/brd.c
> +++ b/drivers/block/brd.c
> @@ -403,7 +403,6 @@ static struct brd_device *brd_alloc(int i)
> disk->flags = GENHD_FL_EXT_DEVT;
> sprintf(disk->disk_name, "ram%d", i);
> set_capacity(disk, rd_size * 2);
> - brd->brd_queue->backing_dev_info->capabilities |= BDI_CAP_SYNCHRONOUS_IO;
>
> /* Tell the block layer that this is not a rotational device */
> blk_queue_flag_set(QUEUE_FLAG_NONROT, brd->brd_queue);
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 9100ac36670afc..d73ddf018fa65f 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -52,6 +52,9 @@ static unsigned int num_devices = 1;
> */
> static size_t huge_class_size;
>
> +static const struct block_device_operations zram_devops;
> +static const struct block_device_operations zram_wb_devops;
> +
> static void zram_free_page(struct zram *zram, size_t index);
> static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> u32 index, int offset, struct bio *bio);
> @@ -408,8 +411,7 @@ static void reset_bdev(struct zram *zram)
> zram->backing_dev = NULL;
> zram->old_block_size = 0;
> zram->bdev = NULL;
> - zram->disk->queue->backing_dev_info->capabilities |=
> - BDI_CAP_SYNCHRONOUS_IO;
> + zram->disk->fops = &zram_devops;
> kvfree(zram->bitmap);
> zram->bitmap = NULL;
> }
> @@ -528,8 +530,7 @@ static ssize_t backing_dev_store(struct device *dev,
> * freely but in fact, IO is going on so finally could cause
> * use-after-free when the IO is really done.
> */
> - zram->disk->queue->backing_dev_info->capabilities &=
> - ~BDI_CAP_SYNCHRONOUS_IO;
> + zram->disk->fops = &zram_wb_devops;
> up_write(&zram->init_lock);

For zram, regardless of BDI_CAP_SYNCHRONOUS_IO, it have used rw_page
every time on read/write path. This one with next patch will make zram
use bio instead of rw_page when it's declared !BDI_CAP_SYNCHRONOUS_IO,
which introduce regression for performance.

In the swap code, BDI_CAP_SYNCHRONOUS_IO is used to avoid swap cache
when the page was private. bdev_read_page is not designed to rely on
synchronous operation. That's why this patch breaks the old behavior.

>
> pr_info("setup backing device %s\n", file_name);
> @@ -1819,6 +1820,13 @@ static const struct block_device_operations zram_devops = {
> .owner = THIS_MODULE
> };
>
> +static const struct block_device_operations zram_wb_devops = {
> + .open = zram_open,
> + .submit_bio = zram_submit_bio,
> + .swap_slot_free_notify = zram_slot_free_notify,
> + .owner = THIS_MODULE
> +};
> +
> static DEVICE_ATTR_WO(compact);
> static DEVICE_ATTR_RW(disksize);
> static DEVICE_ATTR_RO(initstate);
> @@ -1946,8 +1954,7 @@ static int zram_add(void)
> if (ZRAM_LOGICAL_BLOCK_SIZE == PAGE_SIZE)
> blk_queue_max_write_zeroes_sectors(zram->disk->queue, UINT_MAX);
>
> - zram->disk->queue->backing_dev_info->capabilities |=
> - (BDI_CAP_STABLE_WRITES | BDI_CAP_SYNCHRONOUS_IO);
> + zram->disk->queue->backing_dev_info->capabilities |= BDI_CAP_STABLE_WRITES;
> device_add_disk(NULL, zram->disk, zram_disk_attr_groups);
>
> strlcpy(zram->compressor, default_compressor, sizeof(zram->compressor));
> diff --git a/drivers/nvdimm/btt.c b/drivers/nvdimm/btt.c
> index 412d21d8f64351..b4184dc9b41eb4 100644
> --- a/drivers/nvdimm/btt.c
> +++ b/drivers/nvdimm/btt.c
> @@ -1540,8 +1540,6 @@ static int btt_blk_init(struct btt *btt)
> btt->btt_disk->private_data = btt;
> btt->btt_disk->queue = btt->btt_queue;
> btt->btt_disk->flags = GENHD_FL_EXT_DEVT;
> - btt->btt_disk->queue->backing_dev_info->capabilities |=
> - BDI_CAP_SYNCHRONOUS_IO;
>
> blk_queue_logical_block_size(btt->btt_queue, btt->sector_size);
> blk_queue_max_hw_sectors(btt->btt_queue, UINT_MAX);
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 94790e6e0e4ce1..436b83fb24ad61 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -478,7 +478,6 @@ static int pmem_attach_disk(struct device *dev,
> disk->queue = q;
> disk->flags = GENHD_FL_EXT_DEVT;
> disk->private_data = pmem;
> - disk->queue->backing_dev_info->capabilities |= BDI_CAP_SYNCHRONOUS_IO;
> nvdimm_namespace_disk_name(ndns, disk->disk_name);
> set_capacity(disk, (pmem->size - pmem->pfn_pad - pmem->data_offset)
> / 512);
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 52583b6f2ea05d..860ea33571bce5 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -122,9 +122,6 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
> * BDI_CAP_NO_WRITEBACK: Don't write pages back
> * BDI_CAP_NO_ACCT_WB: Don't automatically account writeback pages
> * BDI_CAP_STRICTLIMIT: Keep number of dirty pages below bdi threshold.
> - *
> - * BDI_CAP_SYNCHRONOUS_IO: Device is so fast that asynchronous IO would be
> - * inefficient.
> */
> #define BDI_CAP_NO_ACCT_DIRTY 0x00000001
> #define BDI_CAP_NO_WRITEBACK 0x00000002
> @@ -132,7 +129,6 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
> #define BDI_CAP_STABLE_WRITES 0x00000008
> #define BDI_CAP_STRICTLIMIT 0x00000010
> #define BDI_CAP_CGROUP_WRITEBACK 0x00000020
> -#define BDI_CAP_SYNCHRONOUS_IO 0x00000040
>
> #define BDI_CAP_NO_ACCT_AND_WRITEBACK \
> (BDI_CAP_NO_WRITEBACK | BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_ACCT_WB)
> @@ -174,11 +170,6 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits)
> long congestion_wait(int sync, long timeout);
> long wait_iff_congested(int sync, long timeout);
>
> -static inline bool bdi_cap_synchronous_io(struct backing_dev_info *bdi)
> -{
> - return bdi->capabilities & BDI_CAP_SYNCHRONOUS_IO;
> -}
> -
> static inline bool bdi_cap_stable_pages_required(struct backing_dev_info *bdi)
> {
> return bdi->capabilities & BDI_CAP_STABLE_WRITES;
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 6c26916e95fd4a..18eac97b10e502 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3230,7 +3230,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> if (bdi_cap_stable_pages_required(inode_to_bdi(inode)))
> p->flags |= SWP_STABLE_WRITES;
>
> - if (bdi_cap_synchronous_io(inode_to_bdi(inode)))
> + if (p->bdev && p->bdev->bd_disk->fops->rw_page)
> p->flags |= SWP_SYNCHRONOUS_IO;
>
> if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
> --
> 2.27.0
>

Next message: Randy Dunlap: "Re: [PATCH 0/9] powerpc: delete duplicated words"
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In reply to: Christoph Hellwig: "[PATCH 10/14] bdi: remove BDI_CAP_SYNCHRONOUS_IO"
Next in thread: Christoph Hellwig: "Re: [PATCH 10/14] bdi: remove BDI_CAP_SYNCHRONOUS_IO"
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