Re: remove iomap_writepage v2

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jul 28, 2022 at 01:10:16PM +0200, Jan Kara wrote:
> Hi Christoph!
> 
> On Tue 19-07-22 06:13:07, Christoph Hellwig wrote:
> > this series removes iomap_writepage and it's callers, following what xfs
> > has been doing for a long time.
> 
> So this effectively means "no writeback from page reclaim for these
> filesystems" AFAICT (page migration of dirty pages seems to be handled by
> iomap_migrate_page()) which is going to make life somewhat harder for
> memory reclaim when memory pressure is high enough that dirty pages are
> reaching end of the LRU list. I don't expect this to be a problem on big
> machines but it could have some undesirable effects for small ones
> (embedded, small VMs). I agree per-page writeback has been a bad idea for
> efficiency reasons for at least last 10-15 years and most filesystems
> stopped dealing with more complex situations (like block allocation) from
> ->writepage() already quite a few years ago without any bug reports AFAIK.
> So it all seems like a sensible idea from FS POV but are MM people on board
> or at least aware of this movement in the fs land?

I mentioned it during my folio session at LSFMM, but didn't put a huge
emphasis on it.

For XFS, writeback should already be in progress on other pages if
we're getting to the point of trying to call ->writepage() in vmscan.
Surely this is also true for other filesystems?