On Wed, 14 Nov 2018 17:21:07 -0500 Sasha Levin sashal@kernel.org wrote:
From: Martin Schwidefsky schwidefsky@de.ibm.com
[ 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@redhat.com Tested-by: Li Wang liwang@redhat.com Signed-off-by: Martin Schwidefsky schwidefsky@de.ibm.com Signed-off-by: Sasha Levin sashal@kernel.org
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.