On 25 February 2016 at 16:01, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > On Thu, Feb 25, 2016 at 03:49:33PM +0000, Steve Capper wrote: >> 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. > > 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. Ahhhh, okay, thank you Kirill. I agree, I'll get a patch out. Cheers, -- Steve -- 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>