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enough I guess - NAK. This is not upstreamable, nor anything like it. On Thu, Jul 10, 2025 at 07:57:00PM +0300, Alexey Dobriyan wrote: > On Thu, Jul 10, 2025 at 05:16:52PM +0100, Lorenzo Stoakes wrote: > > Sorry but no - this seems to me to just be a hack. And it also appears to > > violate the rules on BUG_ON() (see [0]) so this is just a no. > > > > [0]:https://lore.kernel.org/linux-mm/CAHk-=wjO1xL_ZRKUG_SJuh6sPTQ-6Lem3a3pGoo26CXEsx_w0g@mail.gmail.com/ > > > > On Wed, Jul 09, 2025 at 09:10:59PM +0300, Alexey Dobriyan wrote: > > > Implement > > > > > > memory.oops_if_bad_pte=1 > > > > This is a totally new paradigm afaict - introducing an oops based on user > > input, I really don't think that's sensible. > > > > Unless kernel.panic_on_oops is set this won't necessarily cause anything to > > halt. Really you want a panic_on_bad_pte here, but that would be way way > > too specific. > > > > So it seems like a hack just so you can get a vmcore? > > > > You seem to be using BUG_ON() to _maybe_ cause a panic, maybe not, but by > > doing this you're inferring that there's unrecoverable system instability, > > which isf clearly not the case. > > > > All of the bad PTE handling seems to be intended to be recoverable and > > handled by calling code. > > > > Additionally we have uses like zap_present_folio_ptes() which use it to > > output PTE state in the instance of an invalid mapcount value - I don't > > think oopsing there would really be what you wanted right? > > > > > > > > boot option which oopses the machine instead of dreadful > > > > > > BUG: Bad page map in process > > > > > > message. > > > > I'm not sure what's so dreadful about it? > > Because the root cause is unknown, happened at unknown time, dmesg > rotated away and nobody bothered to coredump the machine because it > didn't oops! > > > And why an oops is better? > > I apologize for stating the obvious but the less time between the bug > and coredump collection the better. > > > > This is intended > > > for people who want to panic at the slightest provocation and > > > for people who ruled out hardware problems which in turn means that > > > delaying vmcore collection is counter-productive. > > > > Seems to be a specific edge case. > > Yes, but the option is not enabled by default and costs 2 instructions > on the coldest code path, so... > > > > Linux doesn't (never?) panicked on PTE corruption and even implemented > > > ratelimited version of the message meaning it can go for minutes and > > > even hours without anyone noticing which is exactly the opposite of what > > > should be done to facilitate debugging. > > > > But are there real situations you can cite where this has been problematic? > > > > > > > > Not enabled by default. > > > > Yeah, obviously. > > > > > > > > Not advertised. > > > > Umm why? Seems like you just want to add this for your own very specific > > purpose? > > Sort of, I don't want to patch and unpatch things every time. > > > > +/* > > > + * Oops instead of printing "Bad page map in process" message and > > > + * trying to continue. > > > + */ > > > +static bool oops_if_bad_pte __ro_after_init = false; > > > +module_param(oops_if_bad_pte, bool, 0444); > > > + > > > /* > > > * This function is called to print an error when a bad pte > > > * is found. For example, we might have a PFN-mapped pte in > > > @@ -490,6 +498,13 @@ static inline void add_mm_rss_vec(struct mm_struct *mm, int *rss) > > > static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr, > > > pte_t pte, struct page *page) > > > { > > > + /* > > > + * This line is a formality to collect vmcore ASAP. Real bug > > > + * (hardware or software) happened earlier, current registers and > > > + * backtrace aren't interesting. > > > + */ > > > + BUG_ON(oops_if_bad_pte); > > > > Except that it won't without panic_on_oops? > > Yes, I'll update the comment. it is supposed to be used with > panic_on_oops=1 for maximum effect. > > > I mean we can't just go around putting arbitrary BUG_ON()'s like this for > > cases we want data on. > > Yes, we can! > > > And far worse here - this is a print_xxx() function, and you're making it > > oops? That's really bad. > > It's fine because, it is conditional BUG_ON. > > > Note that other page table levels can be 'bad' as well, see pgd_bad() et > > al. - none of these will be caught. > > Sure, I didn't think much about spreading this option to other places. > It can be spread independently. > > > Overall I suspect there's one single case you're worried about, that really > > you want to put a WARN_ON_ONCE() against - then you can panic_on_warn and > > get what you want. > > Ehh, no. WARN is for home users who can maybe photo the oops and fish it > out of dmesg and make bug report -- so that system survives until their > data are flushed to disk. > > I suspect users are very bifurcated: some want to panic always, some > want to panic during QA but not in the field, and then there are users > whose only hope is cellphone camera. > > > If you can make an argument in favour of this that's convincing then that > > would be a potentially upstreamable patch, but this one isn't, in my view.