Re: [f2fs-dev] [f2fs:dev-test 48/48] fs/f2fs/super.c:3303:12: warning: stack frame size of 2064 bytes in function 'f2fs_fill_super'

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jun 05, 2020 at 12:42:59PM -0700, Nick Desaulniers wrote:
> On Fri, Jun 5, 2020 at 12:34 PM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > On Fri, Jun 05, 2020 at 09:45:43AM -0700, Nick Desaulniers wrote:
> > > On Fri, Jun 5, 2020 at 9:08 AM Eric Biggers <ebiggers@kernel.org> wrote:
> > > >
> > > > On Fri, Jun 05, 2020 at 07:55:46AM -0700, Jaegeuk Kim wrote:
> > > > > Eric,
> > > > >
> > > > > Could you PTAL?
> > > > >
> > > > > On 06/05, kernel test robot wrote:
> > > > > > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev-test
> > > > > > head:   adf3d3a53cf13d0998c699ba43d8582c875179e3
> > > > > > commit: adf3d3a53cf13d0998c699ba43d8582c875179e3 [48/48] f2fs: don't return vmalloc() memory from f2fs_kmalloc()
> > > > > > config: powerpc64-randconfig-r011-20200605 (attached as .config)
> > > > > > compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project ac47588bc4ff5927a01ed6fcd269ce86aba52a7c)
> > > > > > reproduce (this is a W=1 build):
> > > > > >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > > > > >         chmod +x ~/bin/make.cross
> > > > > >         # install powerpc64 cross compiling tool for clang build
> > > > > >         # apt-get install binutils-powerpc64-linux-gnu
> > > > > >         git checkout adf3d3a53cf13d0998c699ba43d8582c875179e3
> > > > > >         # save the attached .config to linux build tree
> > > > > >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=powerpc64
> > > > > >
> > > > > > If you fix the issue, kindly add following tag as appropriate
> > > > > > Reported-by: kernel test robot <lkp@intel.com>
> > > >
> > > > I don't know what's causing this.  It doesn't look at all related to my commit,
> > > > and I don't see what's using so much stack in f2fs_fill_super().
> > > >
> > > > @kernel test robot: the directions given above don't actually work.
> > > > First I got an error due to powerpc64-linux-gnu-elfedit not existing.
> > > > So I had to build binutils for powerpc64 myself.
> > > >
> > > > Then I still got an error:
> > > >
> > > >         make: *** No rule to make target 'arch/powerpc64/Makefile'.  Stop.
> > >
> > > If you have the config, then
> > > $ ARCH=powerpc CROSS_COMPILE=powerpc64-linux-gnu- make CC=clang -j71
> > > If you recompile with CONFIG_DEBUG_INFO, you can get the stack frame
> > > information. I wrote a tool to parse this for these cryptic warnings.
> > > https://github.com/ClangBuiltLinux/frame-larger-than
> >
> > I can build the file and get the warning now, but the frame_larger_than.py
> > script doesn't work:
> >
> > $ ARCH=powerpc CROSS_COMPILE=powerpc64-linux-gnu- make CC=clang fs/f2fs/super.o
> >   CALL    scripts/checksyscalls.sh
> >   CALL    scripts/atomic/check-atomics.sh
> >   CC [M]  fs/f2fs/super.o
> > fs/f2fs/super.c:3303:12: warning: stack frame size of 2064 bytes in function 'f2fs_fill_super' [-Wframe-larger-than=]
> > static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
> >            ^
> > 1 warning generated.
> >
> > $ python3 ~/src/frame-larger-than/frame_larger_than.py fs/f2fs/super.o f2fs_fill_super
> > failed to parse elf: Unsupported relocation type: 1
> 
> Looks like the python library I'm using to parse the DWARF debug info
> doesn't understand some ppc64 specific relocation kind, and is
> throwing an ELFError.  Let me report it upstream.
> 
> The hard part for these is inlining; it can be hard to tell which
> child has a large local allocation when looking at the warning about
> the parent.
> 
> Then again, my script is just nicer output than `llvm-dwarfdump foo.o`
> or `readelf --debug-dump=info foo.o` for this specific issue.
> 

I did confirm that my commit "f2fs: don't return vmalloc() memory from
f2fs_kmalloc()" actually caused this, in particular the following part:

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index a71da699cb2d55..f3c15116954218 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -3033,7 +3033,7 @@ static int init_blkz_info(struct f2fs_sb_info *sbi, int devi)
        if (nr_sectors & (bdev_zone_sectors(bdev) - 1))
                FDEV(devi).nr_blkz++;
 
-       FDEV(devi).blkz_seq = f2fs_kzalloc(sbi,
+       FDEV(devi).blkz_seq = f2fs_kvzalloc(sbi,
                                        BITS_TO_LONGS(FDEV(devi).nr_blkz)
                                        * sizeof(unsigned long),
                                        GFP_KERNEL);

This is inlined as:

    f2fs_fill_super()
     => f2fs_scan_devices()
        => init_blkz_info()
           => f2fs_kvzalloc()

Somehow this change increased the frame size of f2fs_fill_super() from 1984 to
2064 bytes.  That exceeds the 2048-byte limit, triggering the warning.

So, f2fs_fill_super() was very close to the limit already.  I'm not sure why; I
don't see any huge stack allocations in it or in static functions it calls.
Maybe there is a kconfig option that is increasing the stack usage?

If I add noinline_for_stack to f2fs_scan_devices(), the frame drops to 1744
bytes.  Or if I add noinline_for_stack to read_raw_super_block(), the frame
drops to 1776 bytes.  We could do either (or both) of those to avoid the
warning.  But we'd still wouldn't be that far from this 2048-byte "limit".

- Eric


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