Re: PATCH [2.4.0test10]: Kiobuf#02, fault-in fix

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Thu, Nov 02, 2000 at 09:30:10AM -0500, Jeff Garzik wrote:
> 
> Dumb question time, if you don't mind.  :)  All code examples are from
> mm/memory.c.
> 
> This seems to imply datain means 'read access':
> 	int datain = (rw == READ);
> 
> And then we pass 'datain' as the 'write_access' arg of handle_mm_fault:
> 	if (handle_mm_fault(current->mm, vma, ptr, datain) <= 0)

Yes.  The kernel often has to make these checks the non-intuitive way
round, because a disk or network read IO actually involves write to
memory, but a write IO only has to read from memory.  The convention
is that read/write flags which affect IO paths indicate whether we are
writing from backing store, so we have to invert the sense to decide
whether it's a write to memory.

> This seems to further imply datain means 'read access':
> 	if (((datain) && (!(vma->vm_flags & VM_WRITE))) ||

No, because the next line is
				err = -EACCES;
so (rw==READ) and !VM_WRITE is an error --- datain does imply write
access to memory.

Cheers,
 Stephen
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