Re: [PATCH V2] powerpc: thp: Fix crash on mremap

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Feb 07, 2014 at 07:21:57PM +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> This patch fix the below crash
> 
> NIP [c00000000004cee4] .__hash_page_thp+0x2a4/0x440
> LR [c0000000000439ac] .hash_page+0x18c/0x5e0
> ...
> Call Trace:
> [c000000736103c40] [00001ffffb000000] 0x1ffffb000000(unreliable)
> [437908.479693] [c000000736103d50] [c0000000000439ac] .hash_page+0x18c/0x5e0
> [437908.479699] [c000000736103e30] [c00000000000924c] .do_hash_page+0x4c/0x58
> 
> On ppc64 we use the pgtable for storing the hpte slot information and
> store address to the pgtable at a constant offset (PTRS_PER_PMD) from
> pmd. On mremap, when we switch the pmd, we need to withdraw and deposit
> the pgtable again, so that we find the pgtable at PTRS_PER_PMD offset
> from new pmd.
> 
> We also want to move the withdraw and deposit before the set_pmd so
> that, when page fault find the pmd as trans huge we can be sure that
> pgtable can be located at the offset.
> 
> variant of upstream SHA1: b3084f4db3aeb991c507ca774337c7e7893ed04f
> for 3.12 stable series

This doesn't look like a "variant", it looks totally different.  Why
can't I just take the b3084f4db3aeb991c507ca774337c7e7893ed04f patch
(and follow-on fix) for 3.12?

I _REALLY_ dislike patches that are totally different from Linus's tree
in stable trees, it has caused nothing but problems in the past.

greg k-h