| Message ID | 20210407014502.24091-34-michel@lespinasse.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC,01/37] mmap locking API: mmap_lock_is_contended returns a bool | expand |
On Tue, Apr 06, 2021 at 06:44:58PM -0700, Michel Lespinasse wrote: > + /* Only try spf for multithreaded user space faults. */ This comment is misleading ... mm_users will also be incremented for ptraced programs as well as programs that are having their /proc/$pid/maps examined, etc. Maybe: /* No need to try spf for single-threaded programs */ Also, please, can we not use an acronym for this feature? It's not a speculative page fault. The page fault is really happening. We're trying to handle it under RCU protection (if anything the faultaround code is the speculative page fault code ...) This is unlocked page fault handling, perhaps? > + if (!(flags & FAULT_FLAG_USER) || atomic_read(&mm->mm_users) == 1) > + goto no_spf; > + > count_vm_event(SPF_ATTEMPT); > seq = mmap_seq_read_start(mm); > if (seq & 1) > @@ -1351,6 +1355,7 @@ void do_user_addr_fault(struct pt_regs *regs, > > spf_abort: > count_vm_event(SPF_ABORT); > +no_spf: > > /* > * Kernel-mode access to the user address space should only occur > -- > 2.20.1 > >
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 48b86911a6df..b1a07ca82d59 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -1318,6 +1318,10 @@ void do_user_addr_fault(struct pt_regs *regs, } #endif + /* Only try spf for multithreaded user space faults. */ + if (!(flags & FAULT_FLAG_USER) || atomic_read(&mm->mm_users) == 1) + goto no_spf; + count_vm_event(SPF_ATTEMPT); seq = mmap_seq_read_start(mm); if (seq & 1) @@ -1351,6 +1355,7 @@ void do_user_addr_fault(struct pt_regs *regs, spf_abort: count_vm_event(SPF_ABORT); +no_spf: /* * Kernel-mode access to the user address space should only occur
Performance tuning: single threaded userspace does not benefit from speculative page faults, so we turn them off to avoid any related (small) extra overheads. Signed-off-by: Michel Lespinasse <michel@lespinasse.org> --- arch/x86/mm/fault.c | 5 +++++ 1 file changed, 5 insertions(+)