Re: [PATCH v2 1/3] pagemap: add mmap-exclusive bit for marking pages mapped only here

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 12.05.2015 15:05, Mark Williamson wrote:
Hi Konstantin,

I hope you won't mind me thinking out loud here on the idea of adding
a flag to the v2 pagemap fields...  From a kernel PoV, I agree that
this seems like the cleanest approach.  However, with my application
developer hat on:

  1. I was hoping we'd be able to backport a compatible fix to older
kernels that might adopt the pagemap permissions change.  Using the V2
format flags rules out doing this for kernels that are too old to have
soft-dirty, I think.
>
  2. From our software's PoV, I feel it's worth noting that it doesn't
strictly fix ABI compatibility, though I realise that's probably not
your primary concern here.  We'll need to modify our code to write the
clear_refs file but that change is OK for us if it's the preferred
solution.

In the patches I've been playing with, I was considering putting the
Exclusive flag in the now-unused PFN field of the pagemap entries.
Since we're specifically trying to work around for the lack of PFN
information, would there be any appetite for mirroring this flag
unconditionally into the now-empty PFN field (i.e. whether using v1 or
v2 flags) when accessed by an unprivileged process?

I realise it's ugly from a kernel PoV and I feel a little bad for
suggesting it - but it would address points 1 and 2 for us (our
existing code just looks for changes in the pagemap entry, so sticking
the flag in there would cause it to do the right thing).

I'm sorry to raise application-specific issues at this point; I
appreciate that your primary concern is to improve the kernel and
technically I like the approach that you've taken!  I'll try and
provide more code-oriented feedback once I've tried out the changes.

I prefer to backport v2 format (except soft-dirty bit and clear_refs)
into older kernels. Page-shift bits are barely used so nobody will see
the difference.

--
Konstantin

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>




[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Asterisk Internet PBX]     [Linux API]