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.
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.
-ss