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. 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. 3. PG_mappedtodisk. This is really only used by the buffer cache. Once the filesystems that use bufferheads have been converted, this can go away. 4. I think I can also consolidate PG_slab and PG_reserved into a "single bit" (not really, but change the encoding so that effectively they only take a single bit). That gives us 4 bits back, which should relieve the pressure on page flag bits for a while. I have Thoughts on PG_private_2 and PG_owner_priv_1, as well as a suspicion that not all combinations of referenced, lru, active, workingset, reclaim and unevictable are possible, and there might be scope for a better encoding. But I don't know that we need to do that work; gaining back 4 bits is already a Big Deal. I'm slowly doing the PG_private transition as part of the folio work. For example, eagle eyed reviewers may have spotted that there is no folio_has_buffers(). Converted code calls folio_buffers() and checks if it's NULL. Help from filesystem maintainers on removing the uses of PG_error gratefully appreciated. [1] https://lwn.net/Articles/894859/
