On 8.12.2017 12:27, Anand Jain wrote:
>
> David,
>
> There are two variants of SB read, one using the device cache [1]
> and the other buffer head [2].
>
> [1] btrfs_read_disk_super()
> [2] btrfs_read_dev_super()
>
> Patch, 6f60cbd3ae442cb35861bb522f388db123d42ec1
> (btrfs: access superblock via pagecache in scan_one_device)
> Embraced device caches to avoid blocksize set and thus avoid device
> drop cache. But however its in the context of starting up with a new
> mount and in practice, would it really matter if the device cache is
> dropped in the context of mount, we any way drop it when the device
> is successfully mounted though. I don't understand the actual problem
> here.
>
> Further [2] is still using buffer head, which works very well for us
> in this context, any idea if there is any suggestion to move it to
> newer bio read instead ?
the bh interface eventually goes:
btrfs_read_dev_one_super
__bread
__bread_gfp
__getblk_gfp
__getblk_slow
__find_get_block
__find_get_block_slow
find_get_page_flags
Which is again using the pagecache, albeit a bit more convoluted.
>
> Thanks, Anand
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