Re: Problems with UVM pagefaults

[Roger Pau Monné <roger.pau%citrix.com@localhost>]

On 09/01/13 00:02, Cherry G. Mathew wrote:
> Hi Roger,
> 
> I've got two immediate thoughts:

Hello Cherry,

Thanks for the feedback

> 
> On 9 January 2013 00:46, Roger Pau Monné <roger.pau%citrix.com@localhost> 
> wrote:
> 
>> This device works by passing a memory region previously allocated with
>> mmap(NULL, size, prot, MAP_ANON | MAP_SHARED, -1, 0). This region is
> 
> i) wire down the mapping with MAP_WIRED
> 
> This will prevent it from being paged in/out and thus pagefaulting on access.

This was a mistake from my previous email, I already allocate the memory
using MAP_WIRED. I've also tried uvm_map_pageable with new_pageable =
FALSE on the memory region, it all ends up with the same result.

> ...
> 
>> So far I've been able to get the ptes, pass them to Xen and stablish the
>> mapping. Writing to that memory area from userspace seems to work fine
>> (using pread), but the problem comes when the userspace program executes
>> something like:
>>
>> pwrite(fd, buf,...)
> 
> ii) If this is a xen specific page aliasing issue (where a writeable
> page is disallowed because it points to RO pagetables mappings of
> domains), perhaps that's nothing to do with UVM ?

Yes, page is RO, but I assume NetBSD will not try to write to that
memory region if what I'm doing is a pwrite, it is supposed to only read
from that memory region and write to the fd.

On the other hand, preads to granted memory pages seems to work ok.