On Thu, Feb 25, 2016 at 03:49:33PM +0000, Steve Capper wrote:

On 23 February 2016 at 18:47, Will Deacon [off-list ref] wrote:

quoted

[adding Steve, since he worked on THP for 32-bit ARM]

Apologies for my late reply...

quoted

On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote:

quoted

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

quoted

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.

FWIW, and since the ARM arch-list is already on cc, I think there is
an issue with pmdp_invalidate() on ARM, since it also seems to clear
the trans_huge (and formerly trans_splitting) bit, which actually makes
the pmd !pmd_present(), but it violates the other requirement from the
comment:
"the pmd_trans_huge and pmd_trans_splitting must remain set at all times
on the pmd until the split is complete for this pmd"

I've only been testing this for arm64 (where I'm yet to see a problem),
but we use the generic pmdp_invalidate implementation from
mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true
after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes
the entire entry... Steve?

pmd_mknotpresent on arm looks inconsistent with the other
architectures and can be changed.

Having had a look at the usage, I can't see it causing an immediate
problem (that needs to be addressed by an emergency patch).
We don't have a notion of splitting pmds (so there is no splitting
information to lose), and the only usage I could see of
pmd_mknotpresent was:

pmdp_invalidate(vma, haddr, pmd);
pmd_populate(mm, pmd, pgtable);

In mm/huge_memory.c, around line 3588.

So we invalidate the entry (which puts down a faulting entry from
pmd_mknotpresent and invalidates tlb), then immediately put down a
table entry with pmd_populate.

I have run a 32-bit ARM test kernel and exacerbated THP splits (that's
what took me time), and I didn't notice any problems with 4.5-rc5.

If I read code correctly, your pmd_mknotpresent() makes the pmd
pmd_none(), right? If yes, it's a problem.

It introduces race I've described here:

https://marc.info/?l=linux-mm&m=144723658100512&w=4

Basically, if zap_pmd_range() would see pmd_none() between
pmdp_mknotpresent() and pmd_populate(), we're screwed.

The race window is small, but it's there.

-- 
 Kirill A. Shutemov

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