Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxx/ Cheers, -- Luís
