Re: [PATCH mm-unstable] mm/khugepaged: fix struct collapse_control load_node definition

["Zach O'Keefe" <zokeefe@google.com>]

On Tue, Sep 6, 2022 at 11:46 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Fri, Sep 2, 2022 at 7:12 PM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > HPAGE_PMD_ORDER is used to determine a minimal amount of bits needed to store
> > per-node counters in struct collapse_control, defining the array to be either a
> > u16[HPAGE_PMD_ORDER] or u32[HPAGE_PMD_ORDER].
> >
> > For some architectures/configs, HPAGE_PMD_ORDER isn't resolved at preprocessor
> > time (suitable for resolving preprocessor directives) but rather expands into a
> > compiler built-in that is resolved at compile-time. E.g. for 32-bit MIPS (see
> > link for full config):
> >
> > ---8<--
> > >> arch/mips/include/asm/pgtable.h:238:26: warning: "__builtin_ffs" is not defined, evaluates to 0 [-Wundef]
> >      238 | #define PTE_T_LOG2      (__builtin_ffs(sizeof(pte_t)) - 1)
> >          |                          ^~~~~~~~~~~~~
> >    arch/mips/include/asm/pgtable-32.h:65:43: note: in expansion of macro 'PTE_T_LOG2'
> >       65 | # define PGDIR_SHIFT    (2 * PAGE_SHIFT - PTE_T_LOG2 - 1)
> >          |                                           ^~~~~~~~~~
> >    include/asm-generic/pgtable-nop4d.h:11:33: note: in expansion of macro 'PGDIR_SHIFT'
> >       11 | #define P4D_SHIFT               PGDIR_SHIFT
> >          |                                 ^~~~~~~~~~~
> >    include/asm-generic/pgtable-nopud.h:18:25: note: in expansion of macro 'P4D_SHIFT'
> >       18 | #define PUD_SHIFT       P4D_SHIFT
> >          |                         ^~~~~~~~~
> >    include/asm-generic/pgtable-nopmd.h:20:25: note: in expansion of macro 'PUD_SHIFT'
> >       20 | #define PMD_SHIFT       PUD_SHIFT
> >          |                         ^~~~~~~~~
> >    include/linux/huge_mm.h:109:25: note: in expansion of macro 'PMD_SHIFT'
> >      109 | #define HPAGE_PMD_SHIFT PMD_SHIFT
> >          |                         ^~~~~~~~~
> >    include/linux/huge_mm.h:105:26: note: in expansion of macro 'HPAGE_PMD_SHIFT'
> >      105 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
> >          |                          ^~~~~~~~~~~~~~~
> >    mm/khugepaged.c:90:5: note: in expansion of macro 'HPAGE_PMD_ORDER'
> >       90 | #if HPAGE_PMD_ORDER < 16
> >          |     ^~~~~~~~~~~~~~~
> > >> arch/mips/include/asm/pgtable.h:238:39: error: missing binary operator before token "("
> >      238 | #define PTE_T_LOG2      (__builtin_ffs(sizeof(pte_t)) - 1)
> >          |                                       ^
> >    arch/mips/include/asm/pgtable-32.h:65:43: note: in expansion of macro 'PTE_T_LOG2'
> >       65 | # define PGDIR_SHIFT    (2 * PAGE_SHIFT - PTE_T_LOG2 - 1)
> >          |                                           ^~~~~~~~~~
> >    include/asm-generic/pgtable-nop4d.h:11:33: note: in expansion of macro 'PGDIR_SHIFT'
> >       11 | #define P4D_SHIFT               PGDIR_SHIFT
> >          |                                 ^~~~~~~~~~~
> >    include/asm-generic/pgtable-nopud.h:18:25: note: in expansion of macro 'P4D_SHIFT'
> >       18 | #define PUD_SHIFT       P4D_SHIFT
> >          |                         ^~~~~~~~~
> >    include/asm-generic/pgtable-nopmd.h:20:25: note: in expansion of macro 'PUD_SHIFT'
> >       20 | #define PMD_SHIFT       PUD_SHIFT
> >          |                         ^~~~~~~~~
> >    include/linux/huge_mm.h:109:25: note: in expansion of macro 'PMD_SHIFT'
> >      109 | #define HPAGE_PMD_SHIFT PMD_SHIFT
> >          |                         ^~~~~~~~~
> >    include/linux/huge_mm.h:105:26: note: in expansion of macro 'HPAGE_PMD_SHIFT'
> >      105 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
> >          |                          ^~~~~~~~~~~~~~~
> >    mm/khugepaged.c:90:5: note: in expansion of macro 'HPAGE_PMD_ORDER'
> >       90 | #if HPAGE_PMD_ORDER < 16
> >          |     ^~~~~~~~~~~~~~~
> > ---8<--
> >
> > load_node is already special-cased for CONFIG_PPC64 in
> > "mm-khugepaged-add-struct-collapse_control-fix-fix", and we could continue down
> > that path by #define'ing away edge cases, but I think at this point it easier
> > to just let load_node be an array of u32s.  If it's ever an issue, we can
> > revisit.
> >
> > This essentially rolls back both:
> >
> > "mm/khugepaged: use minimal bits to store num page < HPAGE_PMD_NR"
> > "mm-khugepaged-add-struct-collapse_control-fix-fix"
> >
> > (though we'll leave it as a u32 vs an int).
> >
> > Link: https://lore.kernel.org/linux-mm/202209021349.F73i5d6X-lkp@intel.com/
> > Fixes: 5f7bb00bd9cd ("mm/khugepaged: use minimal bits to store num page < HPAGE_PMD_NR")
> > Fixes: 03053c821af2 ("mm-khugepaged-add-struct-collapse_control-fix-fix")
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > ---
> >  mm/khugepaged.c | 7 +------
> >  1 file changed, 1 insertion(+), 6 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index af3b07eb2389..29142a35d24c 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -93,13 +93,8 @@ struct collapse_control {
> >         bool is_khugepaged;
> >
> >         /* Num pages scanned per node */
> > -#if defined(CONFIG_PPC64)
> >         u32 node_load[MAX_NUMNODES];
> > -#elif HPAGE_PMD_ORDER < 16
> > -       u16 node_load[MAX_NUMNODES];
> > -#else
> > -       u32 node_load[MAX_NUMNODES];
> > -#endif
>
> I recall we need this to circumvent some build time warning about
> stack usage? Or I misremember?
>

Hey Yang, thanks for keeping a look out here, and good memory!

The stack usage issue was due to struct collapse_control being
allocated on the stack when MAX_NUMNODES was massive[1]. Now that we
kmalloc() them for MADV_COLLAPSE, idea is we'd want to limit required
memory and the original change here was an optimization due to an
observation that we really didn't need 32 bits to hold a maximum value
of HPAGE_PMD_NR[2], but determining some safe upper bound for the
number of requires bits per arch/config as been a bit of a pain, with
special casing adding to support POWER64 already[3], and now this mips
issue that would otherwise require something special.

[1] https://lore.kernel.org/linux-mm/CAAa6QmQXn9odA-dsm-FFKqQmDvBuH0qDkmPFfXDS1so+Loe+4Q@mail.gmail.com/
[2] https://lore.kernel.org/linux-mm/Ys2CeIm%2FQmQwWh9a@google.com/
[3] https://lore.kernel.org/linux-next/Ytk0TQO9khUQgjxT@google.com/