On Wed, Feb 19, 2020 at 01:22:39PM -0800, Andrew Morton wrote: > On Tue, 18 Feb 2020 18:41:51 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > > Jeff Moyer has reported that one of xfstests triggers a warning when run > > on DAX-enabled filesystem: > > > > WARNING: CPU: 76 PID: 51024 at mm/memory.c:2317 wp_page_copy+0xc40/0xd50 > > ... > > wp_page_copy+0x98c/0xd50 (unreliable) > > do_wp_page+0xd8/0xad0 > > __handle_mm_fault+0x748/0x1b90 > > handle_mm_fault+0x120/0x1f0 > > __do_page_fault+0x240/0xd70 > > do_page_fault+0x38/0xd0 > > handle_page_fault+0x10/0x30 > > > > The warning happens on failed __copy_from_user_inatomic() which tries to > > copy data into a CoW page. > > > > This happens because of race between MADV_DONTNEED and CoW page fault: > > > > CPU0 CPU1 > > handle_mm_fault() > > do_wp_page() > > wp_page_copy() > > do_wp_page() > > madvise(MADV_DONTNEED) > > zap_page_range() > > zap_pte_range() > > ptep_get_and_clear_full() > > <TLB flush> > > __copy_from_user_inatomic() > > sees empty PTE and fails > > WARN_ON_ONCE(1) > > clear_page() > > > > The solution is to re-try __copy_from_user_inatomic() under PTL after > > checking that PTE is matches the orig_pte. > > > > The second copy attempt can still fail, like due to non-readable PTE, > > but there's nothing reasonable we can do about, except clearing the CoW > > page. > > You don't think this is worthy of a cc:stable? Please, add it. Although, if I read history correctly, it is 15 year old bug that nobody noticed until we added WARN() there :/ -- Kirill A. Shutemov
