问题
Background : in Java I'm memory mapping a file (shared).
I'm writing some value at the address 0 of that file. I understand the corresponding PAGE in the PAGE CACHE is flagged as DIRTY and will be written later depending on the dirty_ratio and the like settings.
So far so good.
But I'm wondering what is happening when writing once more at the address 0 while the kernel is writing back the dirty page to the file. Is my process blocked somehow waiting for the writeback to be completed?
回答1:
It may be. It is only necessary when the device-level I/O requests include a checksum alongside the written data. Otherwise, the first write may be torn, but it can then be corrected by the second write.
As always, carefully consider your safety against power-failure, kernel crashes etc.
The waiting is allegedly avoided in btrfs. (Also, by happenstance, in the legacy ext3 filesystem. But not ext4 or ext2).
This looks like it is a bit of a moving target. The above (as far as I could tell) describes the first optimization of this "stable page write" code, following the complaints when it was first introduced. The commit description mentions several possibilities for future changes.
- bdi: allow block devices to say that they require stable page writes
- mm: only enforce stable page writes if the backing device requires it
Does my device currently use "stable page writes"?
There is a sysfs attribute you can look at, called stable_pages_required
来源：https://stackoverflow.com/questions/46913388/linux-dirty-page-writeback-and-concurrent-write