On Wed, 2021-08-18 at 17:24 +0800, Miklos Szeredi wrote: > On Tue, 13 Jul 2021 at 04:42, Ed Tsai <ed.tsai@xxxxxxxxxxxx> wrote: > > > > On Tue, 2021-06-08 at 17:30 +0200, Miklos Szeredi wrote: > > > On Thu, 3 Jun 2021 at 14:52, chenguanyou < > > > chenguanyou9338@xxxxxxxxx> > > > wrote: > > > > > > > > ABA deadlock > > > > > > > > PID: 17172 TASK: ffffffc0c162c000 CPU: 6 COMMAND: "Thread-21" > > > > 0 [ffffff802d16b400] __switch_to at ffffff8008086a4c > > > > 1 [ffffff802d16b470] __schedule at ffffff80091ffe58 > > > > 2 [ffffff802d16b4d0] schedule at ffffff8009200348 > > > > 3 [ffffff802d16b4f0] bit_wait at ffffff8009201098 > > > > 4 [ffffff802d16b510] __wait_on_bit at ffffff8009200a34 > > > > 5 [ffffff802d16b5b0] inode_wait_for_writeback at > > > > ffffff800830e1e8 > > > > 6 [ffffff802d16b5e0] evict at ffffff80082fb15c > > > > 7 [ffffff802d16b620] iput at ffffff80082f9270 > > > > 8 [ffffff802d16b680] dentry_unlink_inode at ffffff80082f4c90 > > > > 9 [ffffff802d16b6a0] __dentry_kill at ffffff80082f1710 > > > > 10 [ffffff802d16b6d0] shrink_dentry_list at ffffff80082f1c34 > > > > 11 [ffffff802d16b750] prune_dcache_sb at ffffff80082f18a8 > > > > 12 [ffffff802d16b770] super_cache_scan at ffffff80082d55ac > > > > 13 [ffffff802d16b860] shrink_slab at ffffff8008266170 > > > > 14 [ffffff802d16b900] shrink_node at ffffff800826b420 > > > > 15 [ffffff802d16b980] do_try_to_free_pages at ffffff8008268460 > > > > 16 [ffffff802d16ba60] try_to_free_pages at ffffff80082680d0 > > > > 17 [ffffff802d16bbe0] __alloc_pages_nodemask at > > > > ffffff8008256514 > > > > 18 [ffffff802d16bc60] fuse_copy_fill at ffffff8008438268 > > > > 19 [ffffff802d16bd00] fuse_dev_do_read at ffffff8008437654 > > > > 20 [ffffff802d16bdc0] fuse_dev_splice_read at ffffff8008436f40 > > > > 21 [ffffff802d16be60] sys_splice at ffffff8008315d18 > > > > 22 [ffffff802d16bff0] __sys_trace at ffffff8008084014 > > > > > > > > PID: 9652 TASK: ffffffc0c9ce0000 CPU: 4 COMMAND: > > > > "kworker/u16:8" > > > > 0 [ffffff802e793650] __switch_to at ffffff8008086a4c > > > > 1 [ffffff802e7936c0] __schedule at ffffff80091ffe58 > > > > 2 [ffffff802e793720] schedule at ffffff8009200348 > > > > 3 [ffffff802e793770] __fuse_request_send at ffffff8008435760 > > > > 4 [ffffff802e7937b0] fuse_simple_request at ffffff8008435b14 > > > > 5 [ffffff802e793930] fuse_flush_times at ffffff800843a7a0 > > > > 6 [ffffff802e793950] fuse_write_inode at ffffff800843e4dc > > > > 7 [ffffff802e793980] __writeback_single_inode at > > > > ffffff8008312740 > > > > 8 [ffffff802e793aa0] writeback_sb_inodes at ffffff80083117e4 > > > > 9 [ffffff802e793b00] __writeback_inodes_wb at ffffff8008311d98 > > > > 10 [ffffff802e793c00] wb_writeback at ffffff8008310cfc > > > > 11 [ffffff802e793d00] wb_workfn at ffffff800830e4a8 > > > > 12 [ffffff802e793d90] process_one_work at ffffff80080e4fac > > > > 13 [ffffff802e793e00] worker_thread at ffffff80080e5670 > > > > 14 [ffffff802e793e60] kthread at ffffff80080eb650 > > > > > > The issue is real. > > > > > > The fix, however, is not the right one. The fundamental problem > > > is > > > that fuse_write_inode() blocks on a request to userspace. > > > > > > This is the same issue that fuse_writepage/fuse_writepages > > > face. In > > > that case the solution was to copy the page contents to a > > > temporary > > > buffer and return immediately as if the writeback already > > > completed. > > > > > > Something similar needs to be done here: send the FUSE_SETATTR > > > request > > > asynchronously and return immediately from > > > fuse_write_inode(). The > > > tricky part is to make sure that multiple time updates for the > > > same > > > inode aren't mixed up... > > > > > > Thanks, > > > Miklos > > > > Dear Szeredi, > > > > Writeback thread calls fuse_write_inode() and wait for user Daemon > > to > > complete this write inode request. The user daemon will > > alloc_page() > > after taking this request, and a deadlock could happen when we try > > to > > shrink dentry list under memory pressure. > > > > We (Mediatek) glad to work on this issue for mainline and also LTS. > > So > > another problem is that we should not change the protocol or > > feature > > for stable kernel. > > > > Use GFP_NOFS | __GFP_HIGHMEM can really avoid this by skip the > > dentry > > shirnker. It works but degrade the alloc_page success rate. In a > > more > > fundamental way, we could cache the contents and return > > immediately. > > But how to ensure the request will be done successfully, e.g., > > always > > retry if it fails from daemon. > > Key is where the the dirty metadata is flushed. To prevent deadlock > it must not be flushed from memory reclaim, so must make sure that it > is flushed on close(2) and munmap(2) and not dirtied after that. > > I'm working on this currently and hope to get it ready for the next > merge window. > > Thanks, > Miklos Hi Miklos, I'm not sure whether it has already been resolved in mainline. If it still WIP, please cc me on future emails. Best regards, Ed Tsai
