Re: Freeing page flags

[Matthew Wilcox <willy@infradead.org>]

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.