Re: [PATCH] mm/memory-failure.c: bail out early if huge zero page

[Oscar Salvador <osalvador@suse.de>]

On Tue, Apr 12, 2022 at 05:25:52PM +0800, Miaohe Lin wrote:
> On 2022/4/12 16:31, Oscar Salvador wrote:
> > On Sun, Apr 10, 2022 at 11:22:34PM +0800, Xu Yu wrote:
> >> Kernel panic when injecting memory_failure for the global huge_zero_page,
> >> when CONFIG_DEBUG_VM is enabled, as follows.
> > ... 
> >> In fact, huge_zero_page is unhandlable currently in either soft offline
> >> or memory failure injection.  With CONFIG_DEBUG_VM disabled,
> >> huge_zero_page is bailed out when checking HWPoisonHandlable() in
> >> get_any_page(), or checking page mapping in split_huge_page_to_list().
> >>
> >> This makes huge_zero_page bail out early in madvise_inject_error(), and
> >> panic above won't happen again.
> > 
> > I would not special case this in madvise_inject_error() but rather
> > handle it in memory-failure code.
> > We do already have HWPoisonHandlable(), which tells us whether the page
> > is of a type we can really do something about, so why not add another
> > check in HWPoisonHandlable() for huge_zero_page(), and have that checked
> > in memory_failure().
> 
> IIUC, this does not work. Because HWPoisonHandlable is only called in !MF_COUNT_INCREASED case.
> But MF_COUNT_INCREASED is always set when called from madvise_inject_error, so HWPoisonHandlable
> is not even called in this scene. Or am I miss something?

But nothing stops you from calling it in memory_failure(), right?

 if (MF_COUNT_INCREASED not set) {
  ....
  ...
 } else if(!HWPoisonHandable(p)) {
         action_result(pfn, MF_MSG_UNKNOWN, MF_IGNORED);
         res = -EBUSY;
         goto unlock_mutex;
 }

> BTW: IIRC, LRU isn't set on huge_zero_page. So the origin HWPoisonHandlable can already filter out this page.

I would rather have it as a explicit check than buried in that kind of
assumption.

But after all, Naoya's suggestion might just be better and more focused.


-- 
Oscar Salvador
SUSE Labs