On Tue, 23 Feb 2016 19:19:07 +0100
Gerald Schaefer [off-list ref] wrote:

On Tue, 23 Feb 2016 13:32:21 +0300
"Kirill A. Shutemov" [off-list ref] wrote:

quoted

On Fri, Feb 12, 2016 at 06:16:40PM +0100, Gerald Schaefer wrote:

quoted

On Fri, 12 Feb 2016 16:57:27 +0100
Christian Borntraeger [off-list ref] wrote:

quoted

quoted

I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm?

Don't know, Gerald or Martin?

The implementation frequently changes depending on how many new bits Martin
needs to squeeze out :-)
We don't have a _PAGE_PRESENT bit for pmds, so pmd_present() just checks if the
entry is not empty. pmd_none() of course does the opposite, it checks if it is
empty.

I still worry about pmd_present(). It looks wrong to me. I wounder if
patch below makes a difference.

The theory is that the splitting bit effetely masked bogus pmd_present():
we had pmd_trans_splitting() in all code path and that prevented mm from
touching the pmd. Once pmd_trans_splitting() has gone, mm proceed with the
pmd where it shouldn't and here's a boom.

Well, I don't think pmd_present() == true is bogus for a trans_huge pmd under
splitting, after all there is a page behind the the pmd. Also, if it was
bogus, and it would need to be false, why should it be marked !pmd_present()
only at the pmdp_invalidate() step before the pmd_populate()? It clearly
is pmd_present() before that, on all architectures, and if there was any
problem/race with that, setting it to !pmd_present() at this stage would
only (marginally) reduce the race window.

BTW, PowerPC and Sparc seem to do the same thing in pmdp_invalidate(),
i.e. they do not set pmd_present() == false, only mark it so that it would
not generate a new TLB entry, just like on s390. After all, the function
is called pmdp_invalidate(), and I think the comment in mm/huge_memory.c
before that call is just a little ambiguous in its wording. When it says
"mark the pmd notpresent" it probably means "mark it so that it will not
generate a new TLB entry", which is also what the comment is really about:
prevent huge and small entries in the TLB for the same page at the same
time.

If I am not mistaken this is true for x86 as well. The generic implementation
for pmdp_invalidate sets a new pmd that has been modified with
pmd_mknotpresent. For x86 this function removes the _PAGE_PRESENT and
_PAGE_PROTNONE bits from the entry. The _PAGE_PSE bit stays set and that
makes pmd_present return true.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.