Re: [PATCH] block: make queue limits workable in case of 64K PAGE_SIZE

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jan 01, 2025 at 07:46:41PM -0800, Bart Van Assche wrote:
> On 1/1/25 6:49 PM, Ming Lei wrote:
> > On Wed, Jan 01, 2025 at 06:30:30PM -0800, Bart Van Assche wrote:
> > > Additionally, this patch looks wrong to me. There are good reasons why the
> > > block layer requires that the DMA segment size is at least as large
> > > as the page size.
> > 
> > Do you think 512byte sector can't be DMAed?
> 
> A clarification: I was referring to the maximum DMA segment size. The
> 512 byte number in your email refers to the actual DMA transfer size. My
> statement does not apply to the actual DMA transfer size.

But why does DMA segment size have to be >= PAGE_SIZE(4KB, 64KB)?

I don't think that the constraint is from hardware because DMA controller is
capable of doing DMA on buffer with smaller alignment & size.

IMO, the limit is actually from block layer, that is why I posted out
this patch.

> 
> > > You may want to take a look at this rejected patch series:
> > > Bart Van Assche, "PATCH v6 0/8] Support limits below the page size",
> > > June 2023 (https://lore.kernel.org/linux-block/20230612203314.17820-1-bvanassche@acm.org/).
> > 
> > '502 Bad Gateway' is returned for the above link.
> 
> That link works fine here on multiple devices (laptop, workstation,
> smartphone) so please check your setup.

It is reachable now.

From the link, you have storage controllers with DMA segment size which
is less than 4K, which may never get supported by linux kernel.

I am looking at hardware which works fine on kernel with 4K page size, but
it becomes not workable when kernel is built as 64K page size, which confuses
final kernel users.


Thanks,
Ming