Hi Sergey,
On Fri, Aug 03, 2018 at 11:39:29AM +0900, Sergey Senozhatsky wrote:
On (08/02/18 14:13), Andrew Morton wrote: [..]
That changelog is rather hard to follow. Please review my edits:
: If zram supports writeback feature, it's no longer a BD_CAP_SYNCHRONOUS_IO
^BDI_CAP_SYNCHRONOUS_IO[..]
A reader looking at this would wonder "why the heck are we doing that". Adding a code comment would help them.
The interesting thing here is that include/linux/backing-dev.h BDI_CAP_SYNCHRONOUS_IO comment says
"Device is so fast that asynchronous IO would be inefficient."
Which is not the reason why BDI_CAP_SYNCHRONOUS_IO is used by ZRAM. Probably, the comment needs to be updated as well.
I couldn't catch your point. Could you clarify a little bit more? What do you want to correct for the comment?
Both SWP_SYNCHRONOUS_IO and BDI_CAP_SYNCHRONOUS_IO tend to pivot "efficiency" [looking at the comments], but in ZRAM's case the whole reason to use SYNC IO is a race condition and user-after-free that follows.
Actually, it's not whole reason. As I wrote down, without it, swap_readpage waits the IO completion for a long time by blk_poll so it causes system sluggish problem when device is slow(e.g., zram with backing device).
Thanks.