Re: pahole treats embedded structures a holes

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Thu, May 28, 2026 at 03:58:52PM +0200, Christoph Hellwig wrote:
> On Thu, May 28, 2026 at 10:39:34AM -0300, Arnaldo Carvalho de Melo wrote:
> > > struct rhash_head is a single pointer, so 8 bytes on x86-64, and
> > > xfs_daddr_t is also a 64-bit type, so both the 0 size and the 8
> > > byte hole are clearly wrong.  The kernel .config is attached in case
> > > it matter.

> > Starting from using the BTF info, that becomes available thru sysfs as
> > soon as we load the xfs kernel module:
 
> This works fine on my installed distro kernel as well.  But this is
> for test builds which are only going to run in a VM.  I don't really
> care about DWARF vs BTF, but I do care about not having to run the
> kernel :)

Sure, I was just checking with what I had at hand, before building the
kernel, to see if the problem was there as well.
 
> Note that I'm also seeing this for other code than XFS, e.g. the nvme
> driver or block layer code.
 
> > Now I'll try with a fresh kernel build, with a default fedora kernel
> > config, will take a while, but having access to a separate .o file from
> > the kernel build process, with just DWARF info is what we need to get to
> > the state you're in, that should work, lets see why you're getting the
> > unsatisfactory results you're getting, maybe we need further info about
> > compiler versions, etc, but lets see...
 
> gcc (Debian 15.2.0-17) 15.2.0

acme@number:~$ pahole -C xfs_buf git/build/allmodconfig/fs/xfs/xfs_buf.o  | head
struct xfs_buf {
	struct rhash_head          b_rhash_head;         /*     0     8 */
	xfs_daddr_t                b_rhash_key;          /*     8     8 */
	int                        b_length;             /*    16     4 */

	/* XXX 4 bytes hole, try to pack */

	struct lockref             b_lockref __attribute__((__aligned__(8))); /*    24    80 */
	/* --- cacheline 1 boundary (64 bytes) was 40 bytes ago --- */
	atomic_t                   b_lru_ref __attribute__((__aligned__(4))); /*   104     4 */
acme@number:~$

acme@number:~$ pahole -E -C xfs_buf git/build/allmodconfig/fs/xfs/xfs_buf.o  | head
struct xfs_buf {
	struct rhash_head {
		struct rhash_head * next;                                                /*     0     8 */
	} b_rhash_head; /*     0     8 */
	/* typedef xfs_daddr_t -> __s64 */ long long int              b_rhash_key;       /*     8     8 */
	int                        b_length;                                             /*    16     4 */

	/* XXX 4 bytes hole, try to pack */

	struct lockref {
acme@number:~$

acme@number:~$ readelf -wi git/build/allmodconfig/fs/xfs/xfs_buf.o  | grep -m1 DW_AT_producer
    <e>   DW_AT_producer    : (indirect string, offset: 0x923f): GNU C11 16.1.1 20260515 (Red Hat 16.1.1-2) -march=znver5 -mpopcnt -msse3 -mssse3 -msse4.1 -msse4.2 -mavx2 -mno-fma4 -mno-xop -mfma -mavx512f -mbmi -mbmi2 -maes -mpclmul -mavx512vl -mavx512bw -mavx512dq -mavx512cd -mavx512vbmi -mavx512ifma -mavx512vpopcntdq -mavx512vbmi2 -mgfni -mvpclmulqdq -mavx512vnni -mavx512bitalg -mavx512bf16 -mavx512vp2intersect -madx -mabm -mno-cldemote -mclflushopt -mclwb -mclzero -mcx16 -mno-enqcmd -mf16c -mfsgsbase -mfxsr -mno-hle -msahf -mno-lwp -mlzcnt -mmovbe -mmovdir64b -mmovdiri -mmwaitx -mno-pconfig -mpku -mprfchw -mno-ptwrite -mrdpid -mrdrnd -mrdseed -mno-rtm -mno-serialize -mno-sgx -msha -mshstk -mno-tbm -mno-tsxldtrk -mvaes -mno-waitpkg -mwbnoinvd -mxsave -mxsavec -mxsaveopt -mxsaves -mno-amx-tile -mno-amx-int8 -mno-amx-bf16 -mno-uintr -mno-hreset -mno-kl -mno-widekl -mavxvnni -mno-avx512fp16 -mno-avxifma -mno-avxvnniint8 -mno-avxneconvert -mno-cmpccxadd -mno-amx-fp16 -mprefetchi -mno-raoint -mno-amx-complex -mno-avxvnniint16 -mno-sm3 -mno-sha512 -mno-sm4 -mno-apxf -mno-usermsr -mno-avx10.1 -mno-avx10.2 -mno-amx-avx512 -mno-amx-tf32 -mno-amx-fp8 -mno-movrs -mno-amx-movrs -mno-avx512bmm --param=l1-cache-size=48 --param=l1-cache-line-size=64 --param=l2-cache-size=1024 -mtune=znver5 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a -m64 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mno-red-zone -mcmodel=kernel -mstack-protector-guard-reg=gs -mstack-protector-guard-symbol=__ref_stack_chk_guard -mindirect-branch=thunk-extern -mindirect-branch-register -mindirect-branch-cs-prefix -mfunction-return=thunk-extern -mharden-sls=all -mrecord-mcount -mfentry -mtls-dialect=gnu2 -g -O2 -std=gnu11 -p -fshort-wchar -funsigned-char -fno-common -fno-PIE -fno-strict-aliasing -fms-extensions -fcf-protection=branch -falign-jumps=1 -falign-loops=1 -fno-asynchronous-unwind-tables -fno-jump-tables -fpatchable-function-entry=64,64 -fno-delete-null-pointer-checks -fno-allow-store-data-races -fno-reorder-blocks -fno-ipa-cp-clone -fno-partial-inlining -fstack-protector-strong -ftrivial-auto-var-init=pattern -fzero-init-padding-bits=all -fno-stack-clash-protection -fdiagnostics-show-context=2 -fzero-call-used-regs=used-gpr -fno-inline-functions-called-once -fmin-function-alignment=64 -fstrict-flex-arrays=3 -fno-strict-overflow -fstack-check=no -fconserve-stack -fno-builtin-wcslen -fsanitize=kernel-address -fasan-shadow-offset=0xdffffc0000000000 -fsanitize=bounds-strict -fsanitize=shift -fsanitize=integer-divide-by-zero -fsanitize=bool -fsanitize=enum -fsanitize-coverage=trace-pc -fsanitize-coverage=trace-cmp --param=asan-instrumentation-with-call-threshold=10000 --param=asan-stack=1 --param=asan-instrument-allocas=1 --param=asan-globals=1 --param=asan-kernel-mem-intrinsic-prefix=1
acme@number:~$

then, please take a look at this sequence:

acme@number:~$ readelf -wi git/build/allmodconfig/fs/xfs/xfs_buf.o  | grep -w xfs_buf$ -B1 -A12
 <1><15f59>: Abbrev Number: 76 (DW_TAG_structure_type)
    <15f5a>   DW_AT_name        : (indirect string, offset: 0x8a96): xfs_buf
    <15f5e>   DW_AT_byte_size   : 664
    <15f60>   DW_AT_alignment   : 8
    <15f61>   DW_AT_decl_file   : 49
    <15f62>   DW_AT_decl_line   : 138
    <15f63>   DW_AT_decl_column : 8
    <15f64>   DW_AT_sibling     : <0x1610b>
 <2><15f68>: Abbrev Number: 4 (DW_TAG_member)
    <15f69>   DW_AT_name        : (indirect string, offset: 0x4e54): b_rhash_head
    <15f6d>   DW_AT_decl_file   : 49
    <15f6e>   DW_AT_decl_line   : 146
    <15f6f>   DW_AT_decl_column : 20
    <15f70>   DW_AT_type        : <0x5b22>
acme@number:~$

acme@number:~$ readelf -wi git/build/allmodconfig/fs/xfs/xfs_buf.o  | grep '<5b22>' -A12
 <1><5b22>: Abbrev Number: 30 (DW_TAG_structure_type)
    <5b23>   DW_AT_name        : (indirect string, offset: 0x1bb8): rhash_head
    <5b27>   DW_AT_byte_size   : 8
    <5b28>   DW_AT_decl_file   : 160
    <5b29>   DW_AT_decl_line   : 19
    <5b2a>   DW_AT_decl_column : 8
    <5b2b>   DW_AT_sibling     : <0x5b3d>
 <2><5b2f>: Abbrev Number: 4 (DW_TAG_member)
    <5b30>   DW_AT_name        : (indirect string, offset: 0x123e3): next
    <5b34>   DW_AT_decl_file   : 160
    <5b35>   DW_AT_decl_line   : 20
    <5b36>   DW_AT_decl_column : 28
    <5b37>   DW_AT_type        : <0x5b42>
acme@number:~$

iacme@number:~$ readelf -wi git/build/allmodconfig/fs/xfs/xfs_buf.o  | grep '<5b42>' -A4
 <1><5b42>: Abbrev Number: 8 (DW_TAG_pointer_type)
    <5b43>   DW_AT_byte_size   : 8
    <5b43>   DW_AT_type        : <0x5b22>
 <1><5b47>: Abbrev Number: 30 (DW_TAG_structure_type)
    <5b48>   DW_AT_name        : (indirect string, offset: 0x14c84): rhlist_head
acme@number:~$

So everything is in this CU (debug info for the .o file), at least here,
what do you see on your system?

- Arnaldo