On Mon, Jan 31, 2022 at 03:55:22PM +1100, NeilBrown wrote: > On Mon, 31 Jan 2022, Matthew Wilcox wrote: > > On Mon, Jan 31, 2022 at 03:03:53PM +1100, NeilBrown wrote: > > > - .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE > > > and the flag is set. > > > > Is this actually useful? I ask because Dave Chinner believes > > the call to ->writepage in vmscan to be essentially unused. > > He would be wrong ... unless "essentially" means "mostly" rather than > "totally". > swap-out to NFS results in that ->writepage call. For writes, SWP_FS_OPS uses ->direct_IO, not ->writepage. Confused. > Of course swap_writepage ignores sync_mode, so this might not be > entirely relevant. > > But the main point of the patch is not to return AOP_WRITEPAGE_ACTIVATE > to vmscan. It is to avoid writing at all when WB_SYNC_NONE and > congested. e.g. for POSIX_FADV_DONTNEED > It is also to allow the removal of congestion tracking with minimal > changes to behaviour. > > If I end up changing some dead code into different dead code, I really > don't care. I'm not here to clean up all dead code - only the dead code > specifically related to congestion. > > NeilBrown > > > > See commit 21b4ee7029c9, and I had a followup discussion with him > > on IRC: > > > > <willy> dchinner: did you gather any stats on how often ->writepage was > > being called by pageout() before "xfs: drop ->writepage completely" > > was added? > > <dchinner> willy: Never saw it on XFS in 3 years in my test environment... > > <dchinner> I don't ever recall seeing the memory reclaim guards we put on > > ->writepage in XFS ever firing - IIRC they'd been there for the best > > part of a decade. > > <willy> not so much the WARN_ON firing but the case where it actually calls > > iomap_writepage > > <dchinner> willy: I mean both - I was running with a local patch that warned > > on writepage for a long time, regardless of where it was called from. > > > > I can believe things are different for a network filesystem, or maybe > > XFS does background writeback better than other filesystems, but it > > would be intriguing to be able to get rid of ->writepage altogether > > (or at least from pageout(); migrate.c may be a thornier proposition). > > > >
