On Sat, 13 Feb 2016 01:15:10 +0200 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > I'm trying to wrap my head around the issue and I don't think missing > serialization with gup_fast is the cause -- we just don't need it > anymore. > > Previously, __split_huge_page_splitting() required serialization against > gup_fast to make sure nobody can obtain new reference to the page after > __split_huge_page_splitting() returns. This was a way to stabilize page > references before starting to distribute them from head page to tail > pages. > > With new refcounting, we don't care about this. Splitting PMD is now > decoupled from splitting underlying compound page. It's okay to get new > pins after split_huge_pmd(). To stabilize page references during > split_huge_page() we rely on setting up migration entries once all > pmds are split into page table entries. > > The theory that serialization against gup_fast is not a root cause of the > crashes is consistent no crashes on arm64. Problem is somewhere else. Hmm, ok, I just relied on the commit message of commit fecffad25458, which talks about "pmdp_clear_flush() will do IPI as needed for fast_gup", as well as the comments in mm/gup.c, which also still talk about IPIs and THP splitting. If IPI serialization with fast_gup is not needed anymore for THP splitting, please fix at least the comments in mm/gup.c. > > > > (It also does some some other magic to the attach_count, which might hold off > > > finish_arch_post_lock_switch while some flushing is happening, but this should > > > be unrelated here) > > > > > > > > > > I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm? > > > > > > Don't know, Gerald or Martin? > > > > The implementation frequently changes depending on how many new bits Martin > > needs to squeeze out :-) > > One bit was freed up by the commit you've pointed to as a cause. > I wounder If it's possible that screw up something while removing it? I > don't see it, but who knows. > > Could you check if revert of fecffad25458 helps? I tried reverting fecffad25458, plus re-adding a call to pmdp_splitting_flush() in __split_huge_pmd_locked(), and I could still reproduce the crashes, so I guess it really isn't related to fast_gup vs. THP splitting. > > And could you share how crashes looks like? I haven't seen backtraces yet. > > > We don't have a _PAGE_PRESENT bit for pmds, so pmd_present() just checks if the > > entry is not empty. pmd_none() of course does the opposite, it checks if it is > > empty. > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>