Re: [PATCH 1/2] zram: remove BD_CAP_SYNCHRONOUS_IO with writeback feature

On (08/03/18 11:39), Sergey Senozhatsky wrote:

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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

^ASYNC IO

-ss