Re: [PATCH] fix remap of shared read only mappings

[Jamie Lokier <jamie@shareable.org>]

James Bottomley wrote:
> I think you may misunderstand what I mean by coherence:  Our problem is
> the VIVT processor caches.  Once one mapper does an msync, that data
> must be visible to all the other mappers, so at that point we have to
> flush the cache lines of all the other mappers.  On PA, we only need to
> flush one correctly aligned address to get the VIVT cache to flush all
> the others.  However, the kernel page cache usually holds an unaligned
> reference so we need to do the extra aligned flush when this data
> changes.  If we didn't do the alignment, we'd need to flush every
> virtual address in the current CPU translation for that page.

Ok, I understand why you want matching alignments now. :)
(So that MS_INVALIDATE doesn't have to do anything).

> If you mean PROT_SEM requires immediate coherence without an msync, then
> those semantics would be very tricky to achieve on parisc since we'd
> need the kernel virtual address of the page in the page cache correctly
> aligned as well.

Linux hasn't ever done anything useful with PROT_SEM.  As far as I
know, on some other systems PROT_SEM has meant that you mark pages
uncacheable or similar, but only on systems where that is needed to
implement IPC through the shared memory.  For some definition IPC.

-- JAmie