On Wed, 14 Nov 2018 17:21:07 -0500 Sasha Levin <sashal@xxxxxxxxxx> wrote: > From: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> > > [ Upstream commit e12e4044aede97974f2222eb7f0ed726a5179a32 ] > > In case a fork or a clone system fails in copy_process and the error > handling does the mmput() at the bad_fork_cleanup_mm label, the > following warning messages will appear on the console: > > BUG: non-zero pgtables_bytes on freeing mm: 16384 > > The reason for that is the tricks we play with mm_inc_nr_puds() and > mm_inc_nr_pmds() in init_new_context(). > > A normal 64-bit process has 3 levels of page table, the p4d level and > the pud level are folded. On process termination the free_pud_range() > function in mm/memory.c will subtract 16KB from pgtable_bytes with a > mm_dec_nr_puds() call, but there actually is not really a pud table. > > One issue with this is the fact that pgtable_bytes is usually off > by a few kilobytes, but the more severe problem is that for a failed > fork or clone the free_pgtables() function is not called. In this case > there is no mm_dec_nr_puds() or mm_dec_nr_pmds() that go together with > the mm_inc_nr_puds() and mm_inc_nr_pmds in init_new_context(). > The pgtable_bytes will be off by 16384 or 32768 bytes and we get the > BUG message. The message itself is purely cosmetic, but annoying. > > To fix this override the mm_pmd_folded, mm_pud_folded and mm_p4d_folded > function to check for the true size of the address space. > > Reported-by: Li Wang <liwang@xxxxxxxxxx> > Tested-by: Li Wang <liwang@xxxxxxxxxx> > Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> The fix for the page table accounting problem initially had four git commits: a8874e7e8a "mm: make the __PAGETABLE_PxD_FOLDED defines non-empty" 1071fc5779 "mm: introduce mm_[p4d|pud|pmd]_folded" 6d212db119 "mm: add mm_pxd_folded checks to pgtable_bytes accounting functions" e12e4044ae "s390/mm: fix mis-accounting of pgtable_bytes" In the meantime a fifth git commit is needed to really fix it: 814cedbc0b "s390/mm: correct pgtable_bytes on page table downgrade" The autoselect mechanism only pick up one of the initial four patches. Not good. May I ask *WHY* this patch was picked out of the blue? Same for 4.18-stable. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.
