Re: [PATCH v4 3/7] mm/swap: Add swp_offset_pfn() to fetch PFN from swap entry

[Peter Xu]

On Tue, Aug 23, 2022 at 03:01:09PM -0600, Yu Zhao wrote:
> On Thu, Aug 11, 2022 at 8:33 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >
> > Peter Xu <peterx@xxxxxxxxxx> writes:
> >
> > > We've got a bunch of special swap entries that stores PFN inside the swap
> > > offset fields.  To fetch the PFN, normally the user just calls swp_offset()
> > > assuming that'll be the PFN.
> > >
> > > Add a helper swp_offset_pfn() to fetch the PFN instead, fetching only the
> > > max possible length of a PFN on the host, meanwhile doing proper check with
> > > MAX_PHYSMEM_BITS to make sure the swap offsets can actually store the PFNs
> > > properly always using the BUILD_BUG_ON() in is_pfn_swap_entry().
> > >
> > > One reason to do so is we never tried to sanitize whether swap offset can
> > > really fit for storing PFN.  At the meantime, this patch also prepares us
> > > with the future possibility to store more information inside the swp offset
> > > field, so assuming "swp_offset(entry)" to be the PFN will not stand any
> > > more very soon.
> > >
> > > Replace many of the swp_offset() callers to use swp_offset_pfn() where
> > > proper.  Note that many of the existing users are not candidates for the
> > > replacement, e.g.:
> > >
> > >   (1) When the swap entry is not a pfn swap entry at all, or,
> > >   (2) when we wanna keep the whole swp_offset but only change the swp type.
> > >
> > > For the latter, it can happen when fork() triggered on a write-migration
> > > swap entry pte, we may want to only change the migration type from
> > > write->read but keep the rest, so it's not "fetching PFN" but "changing
> > > swap type only".  They're left aside so that when there're more information
> > > within the swp offset they'll be carried over naturally in those cases.
> > >
> > > Since at it, dropping hwpoison_entry_to_pfn() because that's exactly what
> > > the new swp_offset_pfn() is about.
> > >
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> >
> > LGTM, Thanks!
> >
> > Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> 
> Hi,
> 
> I hit the following crash on mm-everything-2022-08-22-22-59. Please take a look.
> 
> Thanks.
> 
>   kernel BUG at include/linux/swapops.h:117!
>   CPU: 46 PID: 5245 Comm: EventManager_De Tainted: G S         O L
> 6.0.0-dbg-DEV #2
>   RIP: 0010:pfn_swap_entry_to_page+0x72/0xf0
>   Code: c6 48 8b 36 48 83 fe ff 74 53 48 01 d1 48 83 c1 08 48 8b 09 f6
> c1 01 75 7b 66 90 48 89 c1 48 8b 09 f6 c1 01 74 74 5d c3 eb 9e <0f> 0b
> 48 ba ff ff ff ff 03 00 00 00 eb ae a9 ff 0f 00 00 75 13 48
>   RSP: 0018:ffffa59e73fabb80 EFLAGS: 00010282
>   RAX: 00000000ffffffe8 RBX: 0c00000000000000 RCX: ffffcd5440000000
>   RDX: 1ffffffffff7a80a RSI: 0000000000000000 RDI: 0c0000000000042b
>   RBP: ffffa59e73fabb80 R08: ffff9965ca6e8bb8 R09: 0000000000000000
>   R10: ffffffffa5a2f62d R11: 0000030b372e9fff R12: ffff997b79db5738
>   R13: 000000000000042b R14: 0c0000000000042b R15: 1ffffffffff7a80a
>   FS:  00007f549d1bb700(0000) GS:ffff99d3cf680000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000440d035b3180 CR3: 0000002243176004 CR4: 00000000003706e0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   Call Trace:
>    <TASK>
>    change_pte_range+0x36e/0x880
>    change_p4d_range+0x2e8/0x670
>    change_protection_range+0x14e/0x2c0
>    mprotect_fixup+0x1ee/0x330
>    do_mprotect_pkey+0x34c/0x440
>    __x64_sys_mprotect+0x1d/0x30

The VM_BUG_ON added in this patch seems to have revealed a real bug,
because we probably shouldn't call pfn_swap_entry_to_page() upon e.g. a
genuine swap pte.

I'll post a patch shortly, thanks.

-- 
Peter Xu