On Fri, May 13, 2022 at 10:40:05AM +0100, Luís Henriques wrote: > 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/ I remember Jeff asking me about this problem a few days ago. A folio passed to you in ->dirty_folio() or ->invalidate_folio() belongs to your filesystem. Nobody else should be storing to the ->private field; there's no race that could lead to it being freed while you see it. There may, of course, be bugs that are overwriting folio->private, but it's definitely not supposed to happen. I agree with you that it looks like a bit has been set (is it possibly bad RAM?) We do use page->private in the buddy allocator, but that stores the order of the page; it wouldn't be storing 1<<21. PG flag 21 is PG_mlocked, which seems like a weird one to be setting in the wrong field, so probably not that. Is it always bit 21 that gets set?
