On Thu 14-02-19 17:23:39, Michael Ellerman wrote:
> In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT
> rather than just checking that the value is non-zero, e.g.:
>
> static inline int pgd_present(pgd_t pgd)
> {
> - return !pgd_none(pgd);
> + return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
> }
>
> Unfortunately this is broken on big endian, as the result of the
> bitwise && is truncated to int, which is always zero because
> _PAGE_PRESENT is 0x8000000000000000ul. This means pgd_present() and
> pud_present() are always false at compile time, and the compiler
> elides the subsequent code.
>
> Remarkably with that bug present we are still able to boot and run
> with few noticeable effects. However under some work loads we are able
> to trigger a warning in the ext4 code:
Wow, good catch. I wouldn't believe there are so few bad effects from
such a major breakage! :)
Honza
>
> WARNING: CPU: 11 PID: 29593 at fs/ext4/inode.c:3927
> .ext4_set_page_dirty+0x70/0xb0
> CPU: 11 PID: 29593 Comm: debugedit Not tainted 4.20.0-rc1 #1
> ...
> NIP .ext4_set_page_dirty+0x70/0xb0
> LR .set_page_dirty+0xa0/0x150
> Call Trace:
> .set_page_dirty+0xa0/0x150
> .unmap_page_range+0xbf0/0xe10
> .unmap_vmas+0x84/0x130
> .unmap_region+0xe8/0x190
> .__do_munmap+0x2f0/0x510
> .__vm_munmap+0x80/0x110
> .__se_sys_munmap+0x14/0x30
> system_call+0x5c/0x70
>
> The fix is simple, we need to convert the result of the bitwise && to
> an int before returning it.
>
> Thanks to Jan Kara and Aneesh for help with debugging.
>
> Fixes: da7ad366b497 ("powerpc/mm/book3s: Update pmd_present to look at
> _PAGE_PRESENT bit")
> Cc: sta...@vger.kernel.org # v4.20+
> Reported-by: Erhard F. <erhar...@mailbox.org>
> Reviewed-by: Aneesh Kumar K.V <aneesh.ku...@linux.ibm.com>
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
> ---
> arch/powerpc/include/asm/book3s/64/pgtable.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h
> b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index c9bfe526ca9d..d8c8d7c9df15 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -904,7 +904,7 @@ static inline int pud_none(pud_t pud)
>
> static inline int pud_present(pud_t pud)
> {
> - return (pud_raw(pud) & cpu_to_be64(_PAGE_PRESENT));
> + return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PRESENT));
> }
>
> extern struct page *pud_page(pud_t pud);
> @@ -951,7 +951,7 @@ static inline int pgd_none(pgd_t pgd)
>
> static inline int pgd_present(pgd_t pgd)
> {
> - return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
> + return !!(pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT));
> }
>
> static inline pte_t pgd_pte(pgd_t pgd)
> --
> 2.20.1
>
--
Jan Kara <j...@suse.com>
SUSE Labs, CR
