On 6/16/20 3:35 PM, Miklos Szeredi wrote: > On Mon, Jun 15, 2020 at 7:59 AM Vasily Averin <v...@virtuozzo.com> wrote: >> >> On 6/15/20 3:50 AM, kernel test robot wrote: >>> FYI, we noticed the following commit (built with gcc-9): >>> >>> commit: 6b2fb79963fbed7db3ef850926d913518fd5c62f ("fuse: optimize >>> writepages search") >>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master >> >>> [ 1030.995703] ------------[ cut here ]------------ >>> [ 1030.997563] WARNING: CPU: 1 PID: 17211 at fs/fuse/file.c:1728 >>> tree_insert+0xab/0xc0 [fuse] >>> [ 1031.021943] RIP: 0010:tree_insert+0xab/0xc0 [fuse] >>> [ 1031.057802] Call Trace: >>> [ 1031.060015] fuse_writepages_fill+0x5da/0x6a0 [fuse] >>> [ 1031.062536] write_cache_pages+0x171/0x470 >>> [ 1031.064933] ? fuse_writepages+0x100/0x100 [fuse] >>> [ 1031.067419] ? terminate_walk+0xd3/0xf0 >>> [ 1031.069707] ? _cond_resched+0x19/0x30 >>> [ 1031.072140] ? __kmalloc+0x274/0x280 >>> [ 1031.074407] fuse_writepages+0x8a/0x100 [fuse] >>> [ 1031.076599] do_writepages+0x43/0xe0 >> >> It is WARN_ON(!wpa->ia.ap.num_pages); >> however tree_insert() was called from fuse_writepages_fill() with >> ap->num_pages = 0; >> In submitted PATCH RFC we have used >> >> +static int tree_insert(struct rb_root *root, struct fuse_req *ins_req) >> ... >> + pgoff_t idx_to = idx_from + (ins_req->num_pages ? >> + ins_req->num_pages - 1 : 0); >> >> Though committed patch version have >> >> +static void tree_insert(struct rb_root *root, struct fuse_writepage_args >> *wpa) >> ... >> + pgoff_t idx_to = idx_from + wpa->ia.ap.num_pages - 1; >> ... >> + WARN_ON(!wpa->ia.ap.num_pages); >> >> Miklos, >> do you have any objections if I return to our initial proposal? >> Am I missed something and it is not allowed now? > > No objections, but you need to explain with a comment why that special > casing of num_pages == 0 is needed. I don't understand it yet.
fuse_writepages_fill() handles few different cases, in some of them data->wpa should be re-allocated and re-initialized, another cases requires just add new page into existing wpa and increments num_pages. In current implementation fuse_writepages_fill() tries to share the code, by this way for new wpa it calls tree_insert() with num_pages = 0 then switches to common code used non-modified num_pages and increments it at the very end. I've prepared patch that calls tree_insert() with num_pages = 1, It have few other minor optimizations therefore I think it is better than remove incorrect WARN_ON from tree_insert proposed earlier. During investigations I've noticed few other stranges in neighborhood: 1) fuse_writepages() ignores some errors taken from fuse_writepages_fill() I believe it is a bug: if .writepages is called with WB_SYNC_ALL it should either guarantee that all data was successfully saved or return error. 2) fuse_writepages_fill uses following construction: if (wpa && ap->num_pages && (A || B || C)) { action; } else if (wpa && ap->num_pages == data->max_pages) { if (D) { the same action; } } - ap->num_pages check is always true and can be removed - "if" and "else if" calls the same action and can be merged. I'm going to sent separate patches to resolve it. Thank you, Vasily Averin