On Tue, Apr 16, 2024 at 4:07 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote: > > On Tue, Apr 9, 2024 at 5:32 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote: > > > > On Tue, Apr 9, 2024 at 4:46 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Tue, 09 Apr 2024 03:16:20 -0700 syzbot <syzbot+57adb2a4b9d206521bc2@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > Hello, > > > > > > > > syzbot found the following issue on: > > > > > > > > HEAD commit: 8568bb2ccc27 Add linux-next specific files for 20240405 > > > > git tree: linux-next > > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=152f4805180000 > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=48ca5acf8d2eb3bc > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=57adb2a4b9d206521bc2 > > > > compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40 > > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1268258d180000 > > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1256598d180000 > > > > > > Help. From a quick look this seems to be claiming that collapse_file() > > > got to > > > > > > VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); > > > > > > with folio==NULL, but the code look solid regarding this. > > > > > > Given that we have a reproducer, can we expect the bot to perform a > > > bisection for us? > > > > > > > I often don't see a successful automatic bisect, even with > > reproducers. Hit or miss. I will take a closer look tomorrow -- the > > reproducer doesn't look to be doing anything crazy. > > I've only been able to reproduce this using the disk image provided by syzbot. > > What is happening is we are calling MADV_COLLAPSE on an empty mapping > -- which actually reaches collapse_file() -> filemap_lock_folio() > after page_cache_sync_readahead() attempt. This of course fails > correctly, and I can see right before GPF that the returned pointer is > 0xfffffffffffffffe, which is correctly ERR_PTR(-ENOENT). This should > be causing us to take the if (IS_ERR(folio)) {..} path .. but we > don't, and I don't know why. I haven't yet attempted to repro this > against other images. Will continue looking, but wanted to provide > some type of update -- even if it is a disappointing one -- so as to > not appear like I've disappeared. Ugh. Was looking at the wrong source. Thanks hughd@ for mentioning that IS_ERR(folio) changed recently, else I'd have spent more time on it. Fixed by https://lore.kernel.org/all/ZhIWX8K0E2tSyMSr@xxxxxxxxxxxxxxxxxxxx/ > Thanks, > Zach > > > Thanks, > > Zach
