On Fri, 2022-05-13 at 14:38 +0100, Matthew Wilcox wrote: > On Fri, May 13, 2022 at 09:21:11AM -0400, Jeff Layton wrote: > > On Fri, 2022-05-13 at 13:53 +0100, Matthew Wilcox wrote: > > > 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? > > > > No, it varies, but it was always just a few bits in the field that end > > up being set. I was never able to reproduce it locally, but saw it in a > > run in ceph's teuthology lab a few times. Xiubo did the most digging > > here, so he may be able to add more info. > > > > Basically though, we call __filemap_get_folio in netfs_write_begin and > > it will sometimes give us a folio that has PG_private clear, but the > > ->private field has just a few bits that aren't zeroed out. I'm pretty > > sure we zero out that field in ceph, so the theory was that the page was > > traveling through some other subsystem before coming to us. > > It _shouldn't_ be. __filemap_get_folio() may return a page that was > already in the page cache (and so may have had page->private set by > the filesystem originally), or it may allocate a fresh page in > filemap_alloc_folio() which _should_ come with page->private clear. > Adding an assert that is true might be a good debugging tactic: > > +++ b/mm/filemap.c > @@ -2008,6 +2008,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index, > goto repeat; > } > > +VM_BUG_ON_FOLIO(folio->private, folio); > /* > * filemap_add_folio locks the page, and for mmap > * we expect an unlocked page. > > > He wasn't able to ascertain the cause, and just decided to check for > > PG_private instead since you (presumably) shouldn't trust ->private > > unless that's set anyway. > > They are usually in sync ... which means we can reclaim the flag ;-) Agreed. I'm all for freeing up a page bit and PG_private seems like belt-and-suspenders stuff. There are some details in this tracker ticket. This note in particular seems to indicate that ->private is not always coming back as zero: https://tracker.ceph.com/issues/55421#note-20 -- Jeff Layton <jlayton@xxxxxxxxxx>
