Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge or THP

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Nov 30, 2018 at 01:45:46PM +0100, Michal Hocko wrote:
> On Fri 30-11-18 15:36:51, Kirill A. Shutemov wrote:
> > On Fri, Nov 30, 2018 at 01:18:51PM +0100, Michal Hocko wrote:
> > > On Fri 30-11-18 13:06:57, Jan Stancek wrote:
> > > > LTP proc01 testcase has been observed to rarely trigger crashes
> > > > on arm64:
> > > >     page_mapped+0x78/0xb4
> > > >     stable_page_flags+0x27c/0x338
> > > >     kpageflags_read+0xfc/0x164
> > > >     proc_reg_read+0x7c/0xb8
> > > >     __vfs_read+0x58/0x178
> > > >     vfs_read+0x90/0x14c
> > > >     SyS_read+0x60/0xc0
> > > > 
> > > > Issue is that page_mapped() assumes that if compound page is not
> > > > huge, then it must be THP. But if this is 'normal' compound page
> > > > (COMPOUND_PAGE_DTOR), then following loop can keep running
> > > > (for HPAGE_PMD_NR iterations) until it tries to read from memory
> > > > that isn't mapped and triggers a panic:
> > > >         for (i = 0; i < hpage_nr_pages(page); i++) {
> > > >                 if (atomic_read(&page[i]._mapcount) >= 0)
> > > >                         return true;
> > > > 	}
> > > > 
> > > > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> > > > with a custom kernel module [1] which:
> > > > - allocates compound page (PAGEC) of order 1
> > > > - allocates 2 normal pages (COPY), which are initialized to 0xff
> > > >   (to satisfy _mapcount >= 0)
> > > > - 2 PAGEC page structs are copied to address of first COPY page
> > > > - second page of COPY is marked as not present
> > > > - call to page_mapped(COPY) now triggers fault on access to 2nd
> > > >   COPY page at offset 0x30 (_mapcount)
> > > > 
> > > > [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
> > > > 
> > > > Fix the loop to iterate for "1 << compound_order" pages.
> > > 
> > > This is much less magic than the previous version. It is still not clear
> > > to me how is mapping higher order pages to page tables other than THP
> > > though. So a more detailed information about the source would bre really
> > > welcome. Once we know that we can add a Fixes tag and also mark the
> > > patch for stable because that sounds like a stable material.
> > 
> > IIRC, sound subsystem can producuce custom mapped compound pages.
> 
> Do I assume correctly that consecutive ptes simply point to subpages?

Yes. No huge pages there.

-- 
 Kirill A. Shutemov