* Helge Deller (deller@xxxxxx) wrote: > On 11/12/23 02:22, Dr. David Alan Gilbert wrote: > > * matoro (matoro_mailinglist_kernel@xxxxxxxxx) wrote: > > > On 2023-11-11 16:27, Sam James wrote: > > > > Helge Deller <deller@xxxxxx> writes: > > > > > > > > > On 11/11/23 07:31, matoro wrote: > > > > > > Hi Helge, I have bisected a regression in 6.6 which is causing > > > > > > userspace segfaults at a significantly increased rate in kernel 6.6. > > > > > > There seems to be a pathological case triggered by the ninja build > > > > > > tool. The test case I have been using is cmake with ninja backend to > > > > > > attempt to build the nghttp2 package. In 6.6, this segfaults, not at > > > > > > the same location every time, but with enough reliability that I was > > > > > > able to use it as a bisection regression case, including immediately > > > > > > after a reboot. In the kernel log, these show up as "trap #15: Data > > > > > > TLB miss fault" messages. Now these messages can and do show up in > > > > > > 6.5 causing segfaults, but never immediately after a reboot and > > > > > > infrequently enough that the system is stable. With kernel 6.6 I am > > > > > > completely unable to build nghttp2 under any circumstances. > > > > > > > > > > > > I have bisected this down to the following commit: > > > > > > > > > > > > $ git bisect good > > > > > > 3033cd4307681c60db6d08f398a64484b36e0b0f is the first bad commit > > > > > > commit 3033cd4307681c60db6d08f398a64484b36e0b0f > > > > > > Author: Helge Deller <deller@xxxxxx> > > > > > > Date: Sat Aug 19 00:53:28 2023 +0200 > > > > > > > > > > > > parisc: Use generic mmap top-down layout and brk randomization > > > > > > > > > > > > parisc uses a top-down layout by default that exactly fits > > > > > > the generic > > > > > > functions, so get rid of arch specific code and use the > > > > > > generic version > > > > > > by selecting ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. > > > > > > > > > > > > Note that on parisc the stack always grows up and a "unlimited stack" > > > > > > simply means that the value as defined in > > > > > > CONFIG_STACK_MAX_DEFAULT_SIZE_MB > > > > > > should be used. So RLIM_INFINITY is not an indicator to use > > > > > > the legacy > > > > > > memory layout. > > > > > > > > > > > > Signed-off-by: Helge Deller <deller@xxxxxx> > > > > > > > > > > > > arch/parisc/Kconfig | 17 +++++++++++++ > > > > > > arch/parisc/kernel/process.c | 14 ----------- > > > > > > arch/parisc/kernel/sys_parisc.c | 54 > > > > > > +---------------------------------------- > > > > > > mm/util.c | 5 +++- > > > > > > 4 files changed, 22 insertions(+), 68 deletions(-) > > > > > > > > > > Thanks for your report! > > > > > I think it's quite unlikely that this patch introduces such a bad > > > > > regression. > > > > > I'd suspect some other bad commmit, but I'll try to reproduce. > > > > > > > > matoro, does a revert apply cleanly? Does it help? > > > > > > Yes, I just tested this and it cleanly reverts on linux-6.6.y and the revert > > > does fix the issue. > > > > Helge: > > In that patch is: > > > > diff --git a/mm/util.c b/mm/util.c > > index dd12b9531ac4c..8810206444977 100644 > > --- a/mm/util.c > > +++ b/mm/util.c > > @@ -396,7 +396,10 @@ static int mmap_is_legacy(struct rlimit *rlim_stack) > > if (current->personality & ADDR_COMPAT_LAYOUT) > > return 1; > > > > - if (rlim_stack->rlim_cur == RLIM_INFINITY) > > + /* On parisc the stack always grows up - so a unlimited stack should > > + * not be an indicator to use the legacy memory layout. */ > > + if (rlim_stack->rlim_cur == RLIM_INFINITY && > > + !IS_ENABLED(CONFIG_STACK_GROWSUP)) > > return 1; > > > > return sysctl_legacy_va_layout; > > > > is that: > > '!IS_ENABLED(CONFIG_STACK_GROWSUP))' > > > > the right way around? > > > > That feels inverted to me; non-parisc don't have that config > > set, so !IS_ENABLED... is true, so they return 1 instead of checking > > the flag? > > Right. For non-parisc the behaviour didn't change with my patch, and this > is intended. If rlim_stack->rlim_cur == RLIM_INFINITY, non-parisc return 1 as before. > > Note that matoro reported a regression specifically on the parisc platform. Oh, that I missed. > This change: > - if (rlim_stack->rlim_cur == RLIM_INFINITY) > + if (rlim_stack->rlim_cur == RLIM_INFINITY && > + !IS_ENABLED(CONFIG_STACK_GROWSUP)) > just changes the behaviour on parisc. > On parisc rlim_stack->rlim_cur == RLIM_INFINITY" is always true, unless the user > changed the stack limit manually. If unchanged, mmap_is_legacy() should return > sysctl_legacy_va_layout, otherwise 1. > > So, I think that part of the patch is OK. OK, thanks for the clarification. Dave (P.S. and sorry screwing up one email in the header) > Helge -- -----Open up your eyes, open up your mind, open up your code ------- / Dr. David Alan Gilbert | Running GNU/Linux | Happy \ \ dave @ treblig.org | | In Hex / \ _________________________|_____ http://www.treblig.org |_______/
