On Tue, Nov 24, 2020 at 06:16:28PM +0100, Sebastian Andrzej Siewior wrote: > On 2020-11-24 18:52:44 [+0530], Naresh Kamboju wrote: > > While running LTP test case access01 the following kernel BUG > > noticed on linux next 20201124 tag kernel on i386. > > > > git short log: > > ---------------- > > git log --oneline next-20201120..next-20201124 -- mm/highmem.c > > d9927d46febf Merge branch 'akpm-current/current' > > 72d22a0d0e86 mm: support THPs in zero_user_segments > > 2a656cad337e mm/highmem: Take kmap_high_get() properly into account > > > > Please find these easy steps to reproduce the kernel build and boot. > > This BUG_ON() is in zero_user_segments() which ash been added in commit > 72d22a0d0e86 mm: support THPs in zero_user_segments > > > [ 50.852189] kernel BUG at mm/highmem.c:417! > > I managed to capture one invocation with: > zero_user_segments(0xd4367a90, > 0x1000, 0x1000, > 0x0, 0x50) > page_compound() -> 1 > page_size() -> 4096 Thanks for debugging this! I didn't realise start1 was allowed to be less than start2. Try this ... (systemd is sabotaging my efforts to test an i386 kernel) diff --git a/mm/highmem.c b/mm/highmem.c index 3e1087f2b735..6306a535dd9c 100644 --- a/mm/highmem.c +++ b/mm/highmem.c @@ -369,46 +369,39 @@ void zero_user_segments(struct page *page, unsigned start1, unsigned end1, BUG_ON(end1 > page_size(page) || end2 > page_size(page)); for (i = 0; i < compound_nr(page); i++) { - void *kaddr; - unsigned this_end; + void *kaddr = NULL; - if (end1 == 0 && start2 >= PAGE_SIZE) { - start2 -= PAGE_SIZE; - end2 -= PAGE_SIZE; - continue; - } + if (start1 < PAGE_SIZE || start2 < PAGE_SIZE) + kaddr = kmap_atomic(page + i); if (start1 >= PAGE_SIZE) { start1 -= PAGE_SIZE; end1 -= PAGE_SIZE; - if (start2) { - start2 -= PAGE_SIZE; - end2 -= PAGE_SIZE; - } - continue; - } - - kaddr = kmap_atomic(page + i); + } else { + unsigned this_end = min_t(unsigned, end1, PAGE_SIZE); - this_end = min_t(unsigned, end1, PAGE_SIZE); - if (end1 > start1) - memset(kaddr + start1, 0, this_end - start1); - end1 -= this_end; - start1 = 0; + if (end1 > start1) + memset(kaddr + start1, 0, this_end - start1); + end1 -= this_end; + start1 = 0; + } if (start2 >= PAGE_SIZE) { start2 -= PAGE_SIZE; end2 -= PAGE_SIZE; } else { - this_end = min_t(unsigned, end2, PAGE_SIZE); + unsigned this_end = min_t(unsigned, end2, PAGE_SIZE); + if (end2 > start2) memset(kaddr + start2, 0, this_end - start2); end2 -= this_end; start2 = 0; } - kunmap_atomic(kaddr); - flush_dcache_page(page + i); + if (kaddr) { + kunmap_atomic(kaddr); + flush_dcache_page(page + i); + } if (!end1 && !end2) break;