5 ms·

The kernel page cache is an XArray [1] (data structure here [2]) and dirty page pointers are annotated with the dirty tag when the page is dirtied [3]. There is

by afr0ck 3y ago

The kernel page cache is an XArray [1] (data structure here [2]) and dirty page pointers are annotated with the dirty tag when the page is dirtied [3]. There is a per-filesystem callback that usually always ends up here [4], where you can see the the above data structure [2] entry for that page is being tagged as dirty. Edit: fixed typo [1] https://docs.kernel.org/core-api/xarray.html https://docs.kernel.org/core-api/xarray.html [2] https://elixir.bootlin.com/linux/latest/source/include/linux/fs.h#L472 https://elixir.bootlin.com/linux/latest/source/include/linux... [3] https://elixir.bootlin.com/linux/latest/source/mm/page-writeback.c#L2763 https://elixir.bootlin.com/linux/latest/source/mm/page-write... [4] https://elixir.bootlin.com/linux/latest/source/mm/page-writeback.c#L2670 https://elixir.bootlin.com/linux/latest/source/mm/page-write...

tremon 3y ago
Thanks. So there is no "specialized data structure" for tracking dirty pages, the page dirty status is tracked within the regular page cache hierarchy. And the obvious in-line solution (use a wraparound counter for tracking dirty state, increase dirty values for new pages while writing out lower values) isn't easy because the tag is a bitfield. So the easiest solution would be to have two separate dirty markers (e.g. dirty_a and dirty_b), and have the writeout process toggle the new-dirty marker from A to B so it can safely write dirty_a pages without interference. And vice-versa for the next writeout. There is just one caveat with regards to the previous solution: pages that are already pending writeout still need to be locked. In the previous design, a write to a page already pending writeout could be duplicated into the new dirty queue. But since it seems that a page can exist only once in the page cache, it cannot be written to without possibly violating write barriers. So the write must block until the writeout process has un-dirtied the page. Armchair programming is fun, isn't it? :)

by afr0ck 3y ago

tremon 3y ago

Thanks. So there is no "specialized data structure" for tracking dirty pages, the page dirty status is tracked within the regular page cache hierarchy. And the obvious in-line solution (use a wraparound counter for tracking dirty state, increase dirty values for new pages while writing out lower values) isn't easy because the tag is a bitfield. So the easiest solution would be to have two separate dirty markers (e.g. dirty_a and dirty_b), and have the writeout process toggle the new-dirty marker from A to B so it can safely write dirty_a pages without interference. And vice-versa for the next writeout. There is just one caveat with regards to the previous solution: pages that are already pending writeout still need to be locked. In the previous design, a write to a page already pending writeout could be duplicated into the new dirty queue. But since it seems that a page can exist only once in the page cache, it cannot be written to without possibly violating write barriers. So the write must block until the writeout process has un-dirtied the page. Armchair programming is fun, isn't it? :)