Re: [PATCH] block: fix splitting segments

Ming Lei
2020-01-11T12:57:58Z

On Fri, Jan 10, 2020 at 04:36:05AM -0800, Guenter Roeck wrote:
> On 1/9/20 10:37 PM, Ming Lei wrote:
> > On Thu, Jan 09, 2020 at 09:10:24PM -0800, Guenter Roeck wrote:
> > > On 1/9/20 7:00 PM, Ming Lei wrote:
> > > > On Fri, Jan 10, 2020 at 10:58:01AM +0800, Ming Lei wrote:
> > > > > On Thu, Jan 09, 2020 at 08:18:04AM -0700, Jens Axboe wrote:
> > > > > > On 1/9/20 12:16 AM, Christoph Hellwig wrote:
> > > > > > > On Thu, Jan 09, 2020 at 10:03:41AM +0800, Ming Lei wrote:
> > > > > > > > It has been addressed in:
> > > > > > > >
> > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=block-5.5&id=ecd255974caa45901d0b8fab03626e0a18fbc81a
> > > > > > >
> > > > > > > That is probably correct, but still highly suboptimal for most 32-bit
> > > > > > > architectures where physical addresses are 32 bits wide. To fix that
> > > > > > > the proper phys_addr_t type should be used.
> > > > > >
> > > > > > I'll swap it for phys_addr_t - we used to use dma_address_t or something
> > > > > > like that, but I missed this type.
> > > > >
> > > > > Guenter mentioned that 'page_to_phys(start_page) as well as offset are
> > > > > sometimes 0'[1].
> > > > >
> > > > > If that(zero page physical address) can happen when phys_addr_t is 32bit,
> > > > > I guess phys_addr_t may not work too.
> > > > >
> > > > > Guener, could you test the patch in link[2] again?
> > > > >
> > > > >
> > > > > [1] https://lore.kernel.org/linux-block/20200108023822.GB28075@ming.t460p/T/#m5862216b960454fc41a85204defbb887983bfd75
> > > > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=block-5.5&id=b6a89c4a9590663f80486662fe9a9c1f4cee31f4
> > > >
> > > > Loop Guener in.
> > > >
> > >
> > > The patch at [2] doesn't work.
> > >
> > > My understanding is that the page in question is not mapped when
> > > get_max_segment_size() is called (after all, the operation is the
> > > result of a page fault). This is why page_to_phys() returns 0.
> >
> > page_to_phys() supposes to return page's physical address, which
> > should just depend on this machine's physical address space,
> > not related with page mapping.
> >
> > I understand physical address 0 might be valid, such as the 1st
> > page frame of ram.
> >
>
> Not sure if that happens here, but makes sense.
>
> > >
> > > You'll either need a local u64 variable, or use some other means
> > > to handle that situation. Something like
> > >
> > > phys_addr_t paddr = page_to_phys(start_page);
> > >
> > > if (paddr == 0)
> > > return queue_max_segment_size(q);
> > >
> > > at the beginning of the function might do, though there might
> > > still be a problem when the page is later assigned and crosses
> > > a segment boundary (if that is possible).
> >
> > IMO, zero physical address case is the only corner case not
> > covered by using 'phys_addr_t'. If phys_addr_t is 32bit, sum of
> > page_to_phys(start_page) and offset shouldn't be >= 4G.
> >
>
> Yes, but that isn't what is calculated. What is calculated is
> mask - offset + 1
> where
> offset = mask & (page_to_phys(start_page) + offset);
>
> with mask == 0xffffffff, offset == 0, we get:
> mask - offset + 1 = 0xffffffff - 0 + 1 = 0x100000000, which is > 4G.

Yes, queue_max_segment_size() returns 'unsigned int', which is always < 4G,
so min_not_zero() is correct to fix the issue.

Thanks,
Ming