Re: Freeing page flags

[Josef Bacik <josef@toxicpanda.com>]

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.

> 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

Thanks,

Josef