[BUG] bug when enabling VM DEBUG

[Robert Hancock]

On Wed, May 12, 2010 at 7:06 AM, Jamie Lokier <jamie at shareable.org> wrote:
> Catalin Marinas wrote:
>> On Wed, 2010-05-12 at 12:53 +0100, Sebastian Andrzej Siewior wrote:
>> > * Catalin Marinas | 2010-05-12 12:10:39 [+0100]:
>> >
>> > >> > --- a/drivers/ata/libata-sff.c
>> > >> > +++ b/drivers/ata/libata-sff.c
>> > >> > @@ -894,7 +894,7 @@ static void ata_pio_sector(struct ata_queued_cmd *qc)
>> > >> >                                         do_write);
>> > >> >          }
>> > >> >
>> > >> > -       if (!do_write)
>> > >> > +       if (!do_write&&  !PageSlab(page))
>> > >> >                  flush_dcache_page(page);
>> > >>
>> > >> I would think that check belongs inside flush_dcache_page itself, rather
>> > >> than forcing every driver to include it..
>> > >
>> > >Sebastian (cc'ed) reported this as well for MIPS.
>> > Thx. The patch above looks what I've sent a while ago. Jeff was going to
>> > merge it afaik.
>> >
>> > >I think it makes sense for this check to be done in the
>> > >flush_dcache_page() function.
>> >
>> > Why should flush_dcache_page() not flush pages you tell it?
>> > From Documentation/cachetlb.txt:
>> > |   NOTE: This routine need only be called for page cache pages
>> > |          which can potentially ever be mapped into the address
>> > |          space of a user process.  So for example, VFS layer code
>> > |          handling vfs symlinks in the page cache need not call
>> > |          this interface at all.
>> >
>> > A page from slab or stack is not going to see the sky of user land and
>> > therefore it should not be fed into flush_dcache_page().
>>
>> You are right :), so fixing the driver is the best approach.
>
> It worries me that a driver has any knowledge of the PageSlab() flag,
> though.  Especially uncommented knowledge.  That flag seems VM
> internal, and it's conceptually iffy: Kernel code using
> get_free_pages() and using that for I/O also does not see the sky of
> user land.
>
> If all the PIO drivers have to be changed, I'd be happier with:
>
>    flush_dcache_page_for_pio()
>
> which wraps the check, explains it, and provides a single place to
> change if needed.

Indeed.. It's obscure enough for drivers to need to worry about these
details of cache flushing/invalidation at all, let alone for them to
have to worry about even more obscure details..