On Wed, 24 Feb 2021 01:48:07 -0700 Yu Zhao <[email protected]> wrote:
> Currently page_lru() uses Page{Active,Unevictable} to determine which
> lru list a page belongs to. Page{Active,Unevictable} contain
> compound_head() and therefore page_lru() essentially tests
> PG_{active,unevictable} against compound_head(page)->flags. Once an
> lru list is determined, page->lru, rather than
> compound_head(page)->lru, will be added to or deleted from it.
>
> Though not bug, having compound_head() in page_lru() increases the
> size of vmlinux by O(KB) because page_lru() is inlined many places.
> And removing compound_head() entirely from Page{Active,Unevictable}
> may not be the best option (for the moment) either because there
> may be other cases that need compound_head(). This patch makes
> page_lru() and __clear_page_lru_flags(), which are used immediately
> before and after operations on page->lru, test
> PG_{active,unevictable} directly against page->flags instead.
Oh geeze.
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -46,14 +46,12 @@ static __always_inline void __clear_page_lru_flags(struct
> page *page)
> {
> VM_BUG_ON_PAGE(!PageLRU(page), page);
>
> - __ClearPageLRU(page);
> -
> /* this shouldn't happen, so leave the flags to bad_page() */
> - if (PageActive(page) && PageUnevictable(page))
> + if ((page->flags & (BIT(PG_active) | BIT(PG_unevictable))) ==
> + (BIT(PG_active) | BIT(PG_unevictable)))
> return;
This isn't very nice. At the very least we should have (documented!)
helper functions for this:
/* comment goes here */
static inline bool RawPageActive(struct page *page)
{
...
}
However.
Here's what the preprocessor produces for an allmodconfig version of
PageActive():
static inline __attribute__((__gnu_inline__)) __attribute__((__unused__))
__attribute__((no_instrument_function)) __attribute__((__always_inline__)) int
PageActive(struct page *page)
{
return test_bit(PG_active, &({ do { if
(__builtin_expect(!!(PagePoisoned(compound_head(page))), 0)) {
dump_page(compound_head(page), "VM_BUG_ON_PAGE("
"PagePoisoned(compound_head(page))"")"); do { ({ asm volatile("%c0: nop\n\t"
".pushsection .discard.instr_begin\n\t" ".long %c0b - .\n\t" ".popsection\n\t"
: : "i" (373)); }); do { asm volatile("1:\t" ".byte 0x0f, 0x0b" "\n"
".pushsection __bug_table,\"aw\"\n" "2:\t" ".long " "1b" " - 2b" "\t#
bug_entry::bug_addr\n" "\t" ".long " "%c0" " - 2b" "\t# bug_entry::file\n"
"\t.word %c1" "\t# bug_entry::line\n" "\t.word %c2" "\t# bug_entry::flags\n"
"\t.org 2b+%c3\n" ".popsection" : : "i" ("./include/linux/page-flags.h"), "i"
(338), "i" (0), "i" (sizeof(struct bug_entry))); } while (0); do { ({ asm
volatile("%c0:\n\t" ".pushsection .discard.unreachable\n\t" ".long %c0b -
.\n\t" ".popsection\n\t" : : "i" (374)); }); asm volatile("");
__builtin_unreachable(); } while (0); } while (0); } } while (0);
compound_head(page); })->flags);
}
That's all to test a single bit!
Four calls to compound_head().
Compiling this:
int wibble(struct page *page)
{
return PageActive(page);
}
to the below assembly output (allmodconfig build) and it appears that
the compiler did not CSE these calls. Perhaps it would be beneficial
to give it a bit of help.
This is all nuts. How much of this inlining is really justifiable? Do
we know we wouldn't get a better kernel if we did
mv mm-inline.h mm-not-inline-any-more.c
?
Methinks that mm-inline.c needs some serious work...
.type wibble, @function
wibble:
1: call __fentry__
.section __mcount_loc, "a",@progbits
.quad 1b
.previous
pushq %r12 #
pushq %rbp #
pushq %rbx #
# mm/swap.c:1156: {
movq %rdi, %rbx # page, page
movq %rbx, %rbp # page, _14
# ./include/linux/page-flags.h:184: unsigned long head =
READ_ONCE(page->compound_head);
call __sanitizer_cov_trace_pc #
movq 8(%rbx), %r12 # page_2(D)->D.14210.D.14188.compound_head, _8
# ./include/linux/page-flags.h:186: if (unlikely(head & 1))
testb $1, %r12b #, _8
je .L2945 #,
# ./include/linux/page-flags.h:187: return (struct page *) (head -
1);
call __sanitizer_cov_trace_pc #
leaq -1(%r12), %rbp #, _14
jmp .L2945 #
.L2945:
call __sanitizer_cov_trace_pc #
# ./include/linux/page-flags.h:338: PAGEFLAG(Active, active, PF_HEAD)
__CLEARPAGEFLAG(Active, active, PF_HEAD)
cmpq $-1, 0(%rbp) #, MEM[(long unsigned int *)_15]
jne .L2946 #,
# ./include/linux/page-flags.h:184: unsigned long head =
READ_ONCE(page->compound_head);
call __sanitizer_cov_trace_pc #
movq 8(%rbx), %rbp #, _16
# ./include/linux/page-flags.h:186: if (unlikely(head & 1))
testb $1, %bpl #, _16
je .L2947 #,
# ./include/linux/page-flags.h:187: return (struct page *) (head -
1);
leaq -1(%rbp), %rbx #, page
call __sanitizer_cov_trace_pc #
.L2947:
# ./include/linux/page-flags.h:338: PAGEFLAG(Active, active, PF_HEAD)
__CLEARPAGEFLAG(Active, active, PF_HEAD)
call __sanitizer_cov_trace_pc #
movq $.LC20, %rsi #,
movq %rbx, %rdi # page,
call dump_page #
#APP
# 338 "./include/linux/page-flags.h" 1
373: nop #
.pushsection .discard.instr_begin
.long 373b - . #
.popsection
# 0 "" 2
# 338 "./include/linux/page-flags.h" 1
1: .byte 0x0f, 0x0b
.pushsection __bug_table,"aw"
2: .long 1b - 2b # bug_entry::bug_addr
.long .LC3 - 2b # bug_entry::file #
.word 338 # bug_entry::line #
.word 0 # bug_entry::flags #
.org 2b+12 #
.popsection
# 0 "" 2
# 338 "./include/linux/page-flags.h" 1
374: #
.pushsection .discard.unreachable
.long 374b - . #
.popsection
# 0 "" 2
#NO_APP
.L2946:
# ./include/linux/page-flags.h:184: unsigned long head =
READ_ONCE(page->compound_head);
call __sanitizer_cov_trace_pc #
movq 8(%rbx), %rbp #, _28
# ./include/linux/page-flags.h:186: if (unlikely(head & 1))
testb $1, %bpl #, _28
je .L2948 #,
# ./include/linux/page-flags.h:187: return (struct page *) (head -
1);
leaq -1(%rbp), %rbx #, page
call __sanitizer_cov_trace_pc #
.L2948:
# ./arch/x86/include/asm/bitops.h:207: (addr[nr >>
_BITOPS_LONG_SHIFT])) != 0;
call __sanitizer_cov_trace_pc #
movq (%rbx), %rax # MEM[(const long unsigned int *)_35], _24
# mm/swap.c:1158: }
popq %rbx #
popq %rbp #
# ./arch/x86/include/asm/bitops.h:207: (addr[nr >>
_BITOPS_LONG_SHIFT])) != 0;
shrq $5, %rax #, tmp107
# ./include/linux/page-flags.h:338: PAGEFLAG(Active, active, PF_HEAD)
__CLEARPAGEFLAG(Active, active, PF_HEAD)
andl $1, %eax #, tmp106
# mm/swap.c:1158: }
popq %r12 #
ret
