Re: Freeing page flags

["Luís Henriques" <lhenriques@suse.de>]

Matthew Wilcox <willy@infradead.org> writes:

> 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.

In ceph we've recently [1] spent a bit of time debugging a bug related
with ->private not being NULL even though we expected it to be.  The
solution found was to replace the check for NULL and use
folio_test_private() instead, but we _may_ have not figured the whole
thing out.

We assumed that folios were being recycled and not cleaned-up.  The values
we were seeing in ->private looked like they were some sort of flags as
only a few bits were set (e.g. 0x0200000):

[ 1672.578313] page:00000000e23868c1 refcount:2 mapcount:0 mapping:0000000022e0d3b4 index:0xd8 pfn:0x74e83
[ 1672.581934] aops:ceph_aops [ceph] ino:10000016c9e dentry name:"faed"
[ 1672.584457] flags: 0x4000000000000015(locked|uptodate|lru|zone=1)
[ 1672.586878] raw: 4000000000000015 ffffea0001d3a108 ffffea0001d3a088 ffff888003491948
[ 1672.589894] raw: 00000000000000d8 0000000000200000 00000002ffffffff 0000000000000000
[ 1672.592935] page dumped because: VM_BUG_ON_FOLIO(1)

[1] https://lore.kernel.org/all/20220508061543.318394-1-xiubli@redhat.com/

Cheers,
-- 
Luís