Date: Tue, 11 Jun 2019 14:12:22 -0600 From: Alan Somers <asomers@freebsd.org> To: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: panic: vm_fault_hold: fault on nofault entry in fusefs Message-ID: <CAOtMX2gPHy1GWkLyOm5sF=e0zgnj0UEKijFbOnPk6sRo9K4Yew@mail.gmail.com>
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Can somebody please help me to debug a fusefs problem? I have a 100% reproducible panic with the above message. Evidentially there's something I don't know about buf(9) and uiomove(9). The good news is that the panic is sufficiently reproducible and sufficiently instrumented that I know exactly what's happening; I just don't know why. Here's a summary of what happens. 1) fusefs's VOP_WRITE method gets called with a buffer that spans a logical block boundary, but does not extend the size of the file. 2) It splits the write into two parts. Each one calls getblk to allocate a struct buf, fills in the old data with a read, and fills the new data with uiomove. 3) After the file gets close()ed, VOP_INACTIVE calls vn_fsync_buf to flush dirty buffers. 4) VOP_STRATEGY successfully writes the first buffer and frees it with bufdone(). 5) VOP_STRATEGY tries to write the second buffer, but panics during uiomove. The address that caused the panic is always exactly 4KB into the buffer. So what am I doing wrong? The address that causes the panic in step 5 was successfully accessed in step 2, so this isn't some kind of buffer overrun. Does it have something to do with the fact that the read operation in step 2 called bufdone()? Seems unlikely because it did that for both buffers, yet only the second one panics. Or does the address actually fault during both VOP_WRITE and VOP_STRATEGY, but something low down handles the fault in the first case? I'd be grateful for any help that anyone can offer. -Alan P.S. Here's the panic's stack panic: vm_fault_hold: fault on nofault entry, addr: 0xfffffe0004591000 cpuid = 1 time = 1560283621 KDB: stack backtrace: db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0031c21f80 vpanic() at vpanic+0x19d/frame 0xfffffe0031c21fd0 panic() at panic+0x43/frame 0xfffffe0031c22030 vm_fault_hold() at vm_fault_hold+0x2064/frame 0xfffffe0031c22170 vm_fault() at vm_fault+0x60/frame 0xfffffe0031c221b0 trap_pfault() at trap_pfault+0x188/frame 0xfffffe0031c22200 trap() at trap+0x2b4/frame 0xfffffe0031c22310 calltrap() at calltrap+0x8/frame 0xfffffe0031c22310 --- trap 0xc, rip = 0xffffffff8108c9e6, rsp = 0xfffffe0031c223e0, rbp = 0xfffffe0031c223e0 --- memmove_erms() at memmove_erms+0x116/frame 0xfffffe0031c223e0 uiomove_faultflag() at uiomove_faultflag+0x146/frame 0xfffffe0031c22420 fuse_write_directbackend() at fuse_write_directbackend+0x1cd/frame 0xfffffe0031c224f0 fuse_io_strategy() at fuse_io_strategy+0x24d/frame 0xfffffe0031c22590 fuse_vnop_strategy() at fuse_vnop_strategy+0x2a/frame 0xfffffe0031c225a0 VOP_STRATEGY_APV() at VOP_STRATEGY_APV+0x63/frame 0xfffffe0031c225c0 bufstrategy() at bufstrategy+0x44/frame 0xfffffe0031c225f0 bufwrite() at bufwrite+0x259/frame 0xfffffe0031c22640 vn_fsync_buf() at vn_fsync_buf+0x23e/frame 0xfffffe0031c226a0 fuse_vnop_inactive() at fuse_vnop_inactive+0x7e/frame 0xfffffe0031c226e0 VOP_INACTIVE_APV() at VOP_INACTIVE_APV+0x63/frame 0xfffffe0031c22700 vinactive() at vinactive+0xcd/frame 0xfffffe0031c22750 vputx() at vputx+0x2d0/frame 0xfffffe0031c227b0 vn_close1() at vn_close1+0x116/frame 0xfffffe0031c22820 vn_closefile() at vn_closefile+0x4c/frame 0xfffffe0031c228a0 _fdrop() at _fdrop+0x1a/frame 0xfffffe0031c228c0 closef() at closef+0x1ec/frame 0xfffffe0031c22950 closefp() at closefp+0x9c/frame 0xfffffe0031c22990 amd64_syscall() at amd64_syscall+0x276/frame 0xfffffe0031c22ab0 fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe0031c22ab0 --- syscall (6, FreeBSD ELF64, sys_close), rip = 0x8006842ba, rsp = 0x7fffffffe748, rbp = 0x7fffffffe760 --- KDB: enter: panic
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