On Thu, May 12, 2022 at 10:41:41PM -0400, Josef Bacik wrote: > On Thu, May 12, 2022 at 09:54:59PM +0100, Matthew Wilcox wrote: > > The LWN writeup [1] on merging the MGLRU reminded me that I need to send > > out a plan for removing page flags that we can do without. > > > > 1. PG_error. It's basically useless. If the page was read successfully, > > PG_uptodate is set. If not, PG_uptodate is clear. The page cache > > doesn't use PG_error. Some filesystems do, and we need to transition > > them away from using it. > > > > What about writes? A cursory look shows we don't clear Uptodate if we fail to > write, which is correct I think. The only way to indicate we had a write error > to check later is the page error. On encountering a write error, we're supposed to call mapping_set_error(), not SetPageError(). > > 2. PG_private. This tells us whether we have anything stored at > > page->private. We can just check if page->private is NULL or not. > > No need to have this extra bit. Again, there may be some filesystems > > that are a bit wonky here, but I'm sure they're fixable. > > > > At least for Btrfs we serialize the page->private with the private_lock, so we > could probably just drop PG_private, but it's kind of nice to check first before > we have to take the spin lock. I suppose we can just do > > if (page->private) > // do lock and check thingy That's my hope! I think btrfs is already using folio_attach_private() / attach_page_private(), which makes everything easier. Some filesystems still manipulate page->private and PagePrivate by hand.
