On 23 February 2016 at 18:47, Will Deacon <will.deacon@xxxxxxx> wrote: > [adding Steve, since he worked on THP for 32-bit ARM] Apologies for my late reply... > > On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote: >> On Tue, 23 Feb 2016 13:32:21 +0300 >> "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: >> > 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. Cheers, -- Steve > > Will > > -- > 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> -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
