Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)

[John Hubbard]

On 1/23/23 09:33, Matthew Wilcox wrote:
...
> Bleh, I'd forgotten about that problem.  We really do need to keep
> track of which pages are under I/O for this case, because we need to
> tell the filesystem that they are now available for writeback.
>
> That said, I don't know that we need to keep track of it in the
> pages themselves.  Can't we have something similar to rmap which
> keeps track of a range of pinned pages, and have it taken care of
> at a higher level (ie unpin the pages in the dio_iodone_t rather
> than in the BIO completion handler)?
>
> I'm not even sure why pinned pagecache pages remain on the LRU.
> They should probably go onto the unevictable list with the mlocked

This is an intriguing idea, but...

> pages, then on unpin get marked dirty and placed on the active list.
> There's no point in writing back a pinned page since it can be
> written to at any instant without any part of the kernel knowing.

There have been filesystems discussions about this: if a page goes
unwritten for "too long", it's not good. To address that, bounce buffers
were proposed for periodic writeback of pinned pages. The idea with
bounce buffers is: even though the page is never guaranteed up to date
(because DMA can instantly make it effectively dirty), it is at least
much less out of date after a periodic writeback, then it was before.

And that is important for some file system use cases.


thanks,
--
John Hubbard
NVIDIA