On Wed, Oct 19, 2022 at 12:16:36PM +1100, Dave Chinner wrote: > On Wed, Oct 19, 2022 at 09:30:42AM +1100, Dave Chinner wrote: > > On Tue, Oct 18, 2022 at 04:09:17AM +0100, Matthew Wilcox wrote: > > > On Tue, Oct 18, 2022 at 10:52:19AM +0800, Zhaoyang Huang wrote: > > > > On Mon, Oct 17, 2022 at 11:55 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > > > On Mon, Oct 17, 2022 at 01:34:13PM +0800, Zhaoyang Huang wrote: > > > > > > On Fri, Oct 14, 2022 at 8:12 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > On Fri, Oct 14, 2022 at 01:30:48PM +0800, zhaoyang.huang wrote: > > > > > > > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx> > > > > > > > > > > > > > > > > Bellowing RCU stall is reported where kswapd traps in a live lock when shrink > > > > > > > > superblock's inode list. The direct reason is zombie page keeps staying on the > > > > > > > > xarray's slot and make the check and retry loop permanently. The root cause is unknown yet > > > > > > > > and supposed could be an xa update without synchronize_rcu etc. I would like to > > > > > > > > suggest skip this page to break the live lock as a workaround. > > > > > > > > > > > > > > No, the underlying bug should be fixed. > > > > > > > > > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > > Understand. IMHO, find_get_entry actruely works as an open API dealing > > > > with different kinds of address_spaces page cache, which requires high > > > > robustness to deal with any corner cases. Take the current problem as > > > > example, the inode with fault page(refcount=0) could remain on the > > > > sb's list without live lock problem. > > > > > > But it's a corner case that shouldn't happen! What else is going on > > > at the time? Can you reproduce this problem easily? If so, how? > > > > I've been seeing this livelock, too. The reproducer is, > > unfortunately, something I can't share - it's a massive program that > > triggers a data corruption I'm working on solving. > > > > Now that I've > > mostly fixed the data corruption, long duration test runs end up > > livelocking in page cache lookup after several hours. > > > > The test is effectively writing a 100MB file with multiple threads > > doing reverse adjacent racing 1MB unaligned writes. Once the file is > > written, it is then mmap()d and read back from the filesystem for > > verification. > > > > THis is then run with tens of processes concurrently, and then under > > a massively confined memcg (e.g. 32 processes/files are run in a > > memcg with only 200MB of memory allowed). This causes writeback, > > readahead and memory reclaim to race with incoming mmap read faults > > and writes. The livelock occurs on file verification and it appears > > to be an interaction with readahead thrashing. > > > > On my test rig, the physical read to write ratio is at least 20:1 - > > with 32 processes running, the 5s IO rates are: > > > > Device tps MB_read/s MB_wrtn/s MB_dscd/s MB_read MB_wrtn MB_dscd > > dm-0 52187.20 3677.42 1345.92 0.00 18387 6729 0 > > dm-0 62865.60 5947.29 0.08 0.00 29736 0 0 > > dm-0 62972.80 5911.20 0.00 0.00 29556 0 0 > > dm-0 59803.00 5516.72 133.47 0.00 27583 667 0 > > dm-0 63068.20 5292.34 511.52 0.00 26461 2557 0 > > dm-0 56775.60 4184.52 1248.38 0.00 20922 6241 0 > > dm-0 63087.40 5901.26 43.77 0.00 29506 218 0 > > dm-0 62769.00 5833.97 60.54 0.00 29169 302 0 > > dm-0 64810.20 5636.13 305.63 0.00 28180 1528 0 > > dm-0 65222.60 5598.99 349.48 0.00 27994 1747 0 > > dm-0 62444.00 4887.05 926.67 0.00 24435 4633 0 > > dm-0 63812.00 5622.68 294.66 0.00 28113 1473 0 > > dm-0 63482.00 5728.43 195.74 0.00 28642 978 0 > > > > This is reading and writing the same amount of file data at the > > application level, but once the data has been written and kicked out > > of the page cache it seems to require an awful lot more read IO to > > get it back to the application. i.e. this looks like mmap() is > > readahead thrashing severely, and eventually it livelocks with this > > sort of report: > > > > [175901.982484] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: > > [175901.985095] rcu: Tasks blocked on level-1 rcu_node (CPUs 0-15): P25728 > > [175901.987996] (detected by 0, t=97399871 jiffies, g=15891025, q=1972622 ncpus=32) > > [175901.991698] task:test_write state:R running task stack:12784 pid:25728 ppid: 25696 flags:0x00004002 > > [175901.995614] Call Trace: > > [175901.996090] <TASK> > > [175901.996594] ? __schedule+0x301/0xa30 > > [175901.997411] ? sysvec_apic_timer_interrupt+0xb/0x90 > > [175901.998513] ? sysvec_apic_timer_interrupt+0xb/0x90 > > [175901.999578] ? asm_sysvec_apic_timer_interrupt+0x16/0x20 > > [175902.000714] ? xas_start+0x53/0xc0 > > [175902.001484] ? xas_load+0x24/0xa0 > > [175902.002208] ? xas_load+0x5/0xa0 > > [175902.002878] ? __filemap_get_folio+0x87/0x340 > > [175902.003823] ? filemap_fault+0x139/0x8d0 > > [175902.004693] ? __do_fault+0x31/0x1d0 > > [175902.005372] ? __handle_mm_fault+0xda9/0x17d0 > > [175902.006213] ? handle_mm_fault+0xd0/0x2a0 > > [175902.006998] ? exc_page_fault+0x1d9/0x810 > > [175902.007789] ? asm_exc_page_fault+0x22/0x30 > > [175902.008613] </TASK> > > > > Given that filemap_fault on XFS is probably trying to map large > > folios, I do wonder if this is a result of some kind of race with > > teardown of a large folio... > > > > There is a very simple corruption reproducer script that has been > > written, but I haven't been using it. I don't know if long term > > running of the script here: > > > > https://lore.kernel.org/linux-xfs/d00aff43-2bdc-0724-1996-4e58e061ecfd@xxxxxxxxxx/ > > > > will trigger the livelock as the verification step is > > significantly different, but it will give you insight into the > > setup of the environment that leads to the livelock. Maybe you could > > replace the md5sum verification with a mmap read with xfs_io to > > simulate the fault load that seems to lead to this issue... > > FWIW, just tested this on a current Linus kernel. While there is > massive read-ahead thrashing on v6.0, the thrashing is largely gone > in v6.1-rc1+ and the iteration rate of the test is much, much > better. The livelock remains, however. Evidence is starting to point to an interaction with the multi-page folio support in the page cache. I removed the mapping_set_large_folios() calls in the XFS inode instantiation and the test code has now run over 55,000 iterations without failing. The most iterations I'd seen with large folios enabled was about 7,000 - typically it would fail within 2-3,000 iterations. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
