On Thu, Sep 08, 2022 at 08:50:37AM -0400, Brian Geffon wrote:
> Today when a zram device has a backing device we change the ops to
> a new set which does not expose a rw_page method. This prevents the
> upper layers from trying to issue a synchronous rw. This has the
> downside that we penalize every rw even when it could possibly
Do you mean addiontal bio alloc/free?
Please specify something more detail.
> still be performed as a synchronous rw. By the very nature of
Even though zram go though the block layer in the case, it's still
synchronous operation against on in-memory compressed data. Only
asynchrnous IO happens for the data in backing device.
> zram all writes are synchronous so it's unfortunate to have to
> accept this limitation.
>
> This change will always expose a rw_page function and if the page
> has been written back it will return -EOPNOTSUPP which will force the
> upper layers to try again with bio.
Sounds a good idea.
>
> To safely allow a synchronous read to proceed for pages which have not
> yet written back we introduce a new flag ZRAM_NO_WB. On the first
> synchronous read if the page is not written back we will set the
> ZRAM_NO_WB flag. This flag, which is never cleared, prevents writeback
> from ever happening to that page.
Why do we need a addtional flag?
Why couldn't we do?
1. expose the rw_page all the time.
2. If the page was written back, just return an error in rw_page to make
upper layer retry it with bio.
>
> This approach works because in the case of zram as a swap backing device
> the page is going to be removed from zram shortly thereafter so
> preventing writeback is fine. However, if zram is being used as a
> generic block device then this might prevent writeback of the page.
>
> This proposal is still very much RFC, feedback would be appreciated.
>
> Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx>
> ---
> drivers/block/zram/zram_drv.c | 68 +++++++++++++++++++++--------------
> drivers/block/zram/zram_drv.h | 1 +
> 2 files changed, 43 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 92cb929a45b7..22b69e8b6042 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -52,9 +52,6 @@ static unsigned int num_devices = 1;
> static size_t huge_class_size;
>
> static const struct block_device_operations zram_devops;
> -#ifdef CONFIG_ZRAM_WRITEBACK
> -static const struct block_device_operations zram_wb_devops;
> -#endif
>
> static void zram_free_page(struct zram *zram, size_t index);
> static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> @@ -309,7 +306,8 @@ static void mark_idle(struct zram *zram, ktime_t cutoff)
> */
> zram_slot_lock(zram, index);
> if (zram_allocated(zram, index) &&
> - !zram_test_flag(zram, index, ZRAM_UNDER_WB)) {
> + !zram_test_flag(zram, index, ZRAM_UNDER_WB) &&
> + !zram_test_flag(zram, index, ZRAM_NO_WB)) {
> #ifdef CONFIG_ZRAM_MEMORY_TRACKING
> is_idle = !cutoff || ktime_after(cutoff, zram->table[index].ac_time);
> #endif
> @@ -439,7 +437,6 @@ static void reset_bdev(struct zram *zram)
> filp_close(zram->backing_dev, NULL);
> zram->backing_dev = NULL;
> zram->bdev = NULL;
> - zram->disk->fops = &zram_devops;
> kvfree(zram->bitmap);
> zram->bitmap = NULL;
> }
> @@ -543,17 +540,6 @@ static ssize_t backing_dev_store(struct device *dev,
> zram->backing_dev = backing_dev;
> zram->bitmap = bitmap;
> zram->nr_pages = nr_pages;
> - /*
> - * With writeback feature, zram does asynchronous IO so it's no longer
> - * synchronous device so let's remove synchronous io flag. Othewise,
> - * upper layer(e.g., swap) could wait IO completion rather than
> - * (submit and return), which will cause system sluggish.
> - * Furthermore, when the IO function returns(e.g., swap_readpage),
> - * upper layer expects IO was done so it could deallocate the page
> - * freely but in fact, IO is going on so finally could cause
> - * use-after-free when the IO is really done.
> - */
> - zram->disk->fops = &zram_wb_devops;
> up_write(&zram->init_lock);
>
> pr_info("setup backing device %s\n", file_name);
> @@ -722,7 +708,8 @@ static ssize_t writeback_store(struct device *dev,
>
> if (zram_test_flag(zram, index, ZRAM_WB) ||
> zram_test_flag(zram, index, ZRAM_SAME) ||
> - zram_test_flag(zram, index, ZRAM_UNDER_WB))
> + zram_test_flag(zram, index, ZRAM_UNDER_WB) ||
> + zram_test_flag(zram, index, ZRAM_NO_WB))
> goto next;
>
> if (mode & IDLE_WRITEBACK &&
> @@ -1226,6 +1213,10 @@ static void zram_free_page(struct zram *zram, size_t index)
> goto out;
> }
>
> + if (zram_test_flag(zram, index, ZRAM_NO_WB)) {
> + zram_clear_flag(zram, index, ZRAM_NO_WB);
> + }
> +
> /*
> * No memory is allocated for same element filled pages.
> * Simply clear same page flag.
> @@ -1654,6 +1645,40 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
> index = sector >> SECTORS_PER_PAGE_SHIFT;
> offset = (sector & (SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
>
> +#ifdef CONFIG_ZRAM_WRITEBACK
> + /*
> + * With writeback feature, zram does asynchronous IO so it's no longer
> + * synchronous device so let's remove synchronous io flag. Othewise,
> + * upper layer(e.g., swap) could wait IO completion rather than
> + * (submit and return), which will cause system sluggish.
> + * Furthermore, when the IO function returns(e.g., swap_readpage),
> + * upper layer expects IO was done so it could deallocate the page
> + * freely but in fact, IO is going on so finally could cause
> + * use-after-free when the IO is really done.
> + *
> + * If the page is not currently written back then we may proceed to
> + * read the page synchronously, otherwise, we must fail with
> + * -EOPNOTSUPP to force the upper layers to use a normal bio.
> + */
> + zram_slot_lock(zram, index);
> + if (zram_test_flag(zram, index, ZRAM_WB) ||
> + zram_test_flag(zram, index, ZRAM_UNDER_WB)) {
> + zram_slot_unlock(zram, index);
> + /* We cannot proceed with synchronous read */
> + return -EOPNOTSUPP;
> + }
> +
> + /*
> + * Don't allow the page to be written back while we read it,
> + * this flag is never cleared. It shouldn't be a problem that
> + * we don't clear this flag because in the case of swap this
> + * page will be removed shortly after this read anyway.
> + */
> + if (op == REQ_OP_READ)
> + zram_set_flag(zram, index, ZRAM_NO_WB);
> + zram_slot_unlock(zram, index);
> +#endif
> +
> bv.bv_page = page;
> bv.bv_len = PAGE_SIZE;
> bv.bv_offset = 0;
> @@ -1827,15 +1852,6 @@ static const struct block_device_operations zram_devops = {
> .owner = THIS_MODULE
> };
>
> -#ifdef CONFIG_ZRAM_WRITEBACK
> -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
> -};
> -#endif
> -
> static DEVICE_ATTR_WO(compact);
> static DEVICE_ATTR_RW(disksize);
> static DEVICE_ATTR_RO(initstate);
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 158c91e54850..20e4c6a579e0 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -50,6 +50,7 @@ enum zram_pageflags {
> ZRAM_UNDER_WB, /* page is under writeback */
> ZRAM_HUGE, /* Incompressible page */
> ZRAM_IDLE, /* not accessed page since last idle marking */
> + ZRAM_NO_WB, /* Do not allow page to be written back */
>
> __NR_ZRAM_PAGEFLAGS,
> };
> --
> 2.37.2.789.g6183377224-goog
>
>
