Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings

[Mike Kravetz]

On 08/15/22 20:38, Gerald Schaefer wrote:
> On Mon, 15 Aug 2022 20:03:20 +0200
> David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On Mon, Aug 15, 2022 at 5:59 PM Gerald Schaefer
> > <gerald.schaefer@xxxxxxxxxxxxx> wrote:
> > > On Mon, 15 Aug 2022 17:07:32 +0200
> > > David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > > On Mon, Aug 15, 2022 at 3:36 PM Gerald Schaefer
> > > > <gerald.schaefer@xxxxxxxxxxxxx> wrote:
> > > > > On Thu, 11 Aug 2022 11:59:09 -0700
> > > > > Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > > > >
> > > Sure, forgot to send it with initial reply...
> > >
> > > [   82.574749] ------------[ cut here ]------------
> > > [   82.574751] WARNING: CPU: 9 PID: 1674 at mm/hugetlb.c:5264 hugetlb_wp+0x3be/0x818
> > > [   82.574759] Modules linked in: nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nf_tables nfnetlink sunrpc uvdevice s390_trng vfio_ccw mdev vfio_iommu_type1 eadm_sch vfio zcrypt_cex4 sch_fq_codel configfs ghash_s390 prng chacha_s390 libchacha aes_s390 des_s390 libdes sha3_512_s390 sha3_256_s390 sha512_s390 sha256_s390 sha1_s390 sha_common pkey zcrypt rng_core autofs4
> > > [   82.574785] CPU: 9 PID: 1674 Comm: linkhuge_rw Kdump: loaded Not tainted 5.19.0-next-20220815 #36
> > > [   82.574787] Hardware name: IBM 3931 A01 704 (LPAR)
> > > [   82.574788] Krnl PSW : 0704c00180000000 00000006c9d4bc6a (hugetlb_wp+0x3c2/0x818)
> > > [   82.574791]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
> > > [   82.574794] Krnl GPRS: 000000000227c000 0000000008640071 0000000000000000 0000000001200000
> > > [   82.574796]            0000000001200000 00000000b5a98090 0000000000000255 00000000adb2c898
> > > [   82.574797]            0000000000000000 00000000adb2c898 0000000001200000 00000000b5a98090
> > > [   82.574799]            000000008c408000 0000000092fd7300 000003800339bc10 000003800339baf8
> > > [   82.574803] Krnl Code: 00000006c9d4bc5c: f160000407fe        mvo     4(7,%r0),2046(1,%r0)
> > >            00000006c9d4bc62: 47000700           bc      0,1792
> > >           #00000006c9d4bc66: af000000           mc      0,0
> > >           >00000006c9d4bc6a: a7a80040           lhi     %r10,64
> > >            00000006c9d4bc6e: b916002a           llgfr   %r2,%r10
> > >            00000006c9d4bc72: eb6ff1600004       lmg     %r6,%r15,352(%r15)
> > >            00000006c9d4bc78: 07fe               bcr     15,%r14
> > >            00000006c9d4bc7a: 47000700           bc      0,1792
> > > [   82.574814] Call Trace:
> > > [   82.574842]  [<00000006c9d4bc6a>] hugetlb_wp+0x3c2/0x818
> > > [   82.574846]  [<00000006c9d4c62e>] hugetlb_no_page+0x56e/0x5a8
> > > [   82.574848]  [<00000006c9d4cac2>] hugetlb_fault+0x45a/0x590
> > > [   82.574850]  [<00000006c9d06d4a>] handle_mm_fault+0x182/0x220
> > > [   82.574855]  [<00000006c9a9d70e>] do_exception+0x19e/0x470
> > > [   82.574858]  [<00000006c9a9dff2>] do_dat_exception+0x2a/0x50
> > > [   82.574861]  [<00000006ca668a18>] __do_pgm_check+0xf0/0x1b0
> > > [   82.574866]  [<00000006ca677b3c>] pgm_check_handler+0x11c/0x170
> > > [   82.574870] Last Breaking-Event-Address:
> > > [   82.574871]  [<00000006c9d4b926>] hugetlb_wp+0x7e/0x818
> > > [   82.574873] Kernel panic - not syncing: panic_on_warn set ...
> > > [   82.574875] CPU: 9 PID: 1674 Comm: linkhuge_rw Kdump: loaded Not tainted 5.19.0-next-20220815 #36
> > > [   82.574877] Hardware name: IBM 3931 A01 704 (LPAR)
> > > [   82.574878] Call Trace:
> > > [   82.574879]  [<00000006ca664f22>] dump_stack_lvl+0x62/0x80
> > > [   82.574881]  [<00000006ca657af8>] panic+0x118/0x300
> > > [   82.574884]  [<00000006c9ac3da6>] __warn+0xb6/0x160
> > > [   82.574887]  [<00000006ca29b1ea>] report_bug+0xba/0x140
> > > [   82.574890]  [<00000006c9a75194>] monitor_event_exception+0x44/0x80
> > > [   82.574892]  [<00000006ca668a18>] __do_pgm_check+0xf0/0x1b0
> > > [   82.574894]  [<00000006ca677b3c>] pgm_check_handler+0x11c/0x170
> > > [   82.574897]  [<00000006c9d4bc6a>] hugetlb_wp+0x3c2/0x818
> > > [   82.574899]  [<00000006c9d4c62e>] hugetlb_no_page+0x56e/0x5a8
> > > [   82.574901]  [<00000006c9d4cac2>] hugetlb_fault+0x45a/0x590
> > > [   82.574903]  [<00000006c9d06d4a>] handle_mm_fault+0x182/0x220
> > > [   82.574906]  [<00000006c9a9d70e>] do_exception+0x19e/0x470
> > > [   82.574907]  [<00000006c9a9dff2>] do_dat_exception+0x2a/0x50
> > > [   82.574909]  [<00000006ca668a18>] __do_pgm_check+0xf0/0x1b0
> > > [   82.574912]  [<00000006ca677b3c>] pgm_check_handler+0x11c/0x170
> > 
> > 
> > do_dat_exception() sets
> >   access = VM_ACCESS_FLAGS;
> > 
> > do_exception() sets
> >   is_write = (trans_exc_code & store_indication) == 0x400;
> > 
> > and FAULT_FLAG_WRITE
> >    if (access == VM_WRITE || is_write)
> >           flags |= FAULT_FLAG_WRITE;
> > 
> > however, for VMA permission checks it only checks
> >   if (unlikely(!(vma->vm_flags & access)))
> >           goto out_up;
> > 
> > as VM_ACCESS_FLAGS includes VM_WRITE | VM_READ ...
> > 
> > We end up triggering a write fault (FAULT_FLAG_WRITE), even though the
> > VMA does not allow for writes.
> > 
> > I assume that's what happens and that it's a bug in s390x code.
> > 
> 
> Hmm, that looks weird, but that doesn't mean it has to be broken.
> We are talking about a pte_none() fault, not a protection exception
> (do_dat_exception vs. do_protection_exception). Not sure if we get
> any proper store indication in that case, but yes, this looks weird,
> will have a closer look. Thanks for pointing out!
> 
> FWIW, meanwhile, I added a check to hugetlb_wp() in v5.19, for
> (!unshare && !(vma->vm_flags & VM_WRITE)). This did not trigger,
> however, it did trigger already before your commit. So something
> already changed before your commit, and after v5.19.
> 
> Further bisecting showed that the check started to trigger
> after commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()"),
> and after that the "HUGETLB_ELFMAP=R linkhuge_rw" testcase also
> started segfaulting (not sure why we did not notice earlier...).
> 
> Anyway, I guess this means that your commit only made that change
> in behavior more obvious, by adding the WARN_ON_ONCE, but it really
> was introduced by that other commit.
> 
> Not sure if this gives any more insight to anyone, still confused
> by your comments on do_exception(), which also sound like a possible
> root cause for ending up in hugetlb_wp() w/o VM_WRITE (but why only
> after commit bcd51a3c679d?).

I know it doesn't mean much, but I did not/do not see these issues on x86.

bcd51a3c679d eliminates the copying of page tables at fork for non-anon
hugetlb vmas.  So, in these tests you would likely see more pte_none()
faults.

I'm trying to figure out exactly what this test is doing.  From a quick
look it is doing a fork/write fault to determine if addresses are writable.
Guessing that such faults are triggering the warning.  Will look at this
some more to try and gain more insight.
-- 
Mike Kravetz