lkml.org 
[lkml]   [2021]   [Feb]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    Subject[PATCH v2 1/3] mm: bypass compound_head() for PF_NO_TAIL when enforce=1
    From
    When testing page flags with PF_NO_TAIL (enforce=0), tail pages are
    legit and they are converted by compound_head(). When modifying page
    flags (enforce=1), tail pages are not legit and they either trigger
    VM_BUG_ON_PGFLAGS() or are "corrected" by compound_head().

    There is no evidence such "correction" is beneficial in terms of
    preventing a buggy kernel from crashing. But there is clear evidence
    it's detrimental to the size of vmlinux because compound_head() is
    redundantly inlined for all modifications of small and head page flags
    using PF_NO_TAIL.

    This patch makes PF_NO_TAIL skip compound_head() when modifying page
    flags. There won't be any behavior change unless a piece of buggy code
    tries to modify tail page flags using PF_NO_TAIL. Such code won't be
    "corrected", if VM_BUG_ON_PGFLAGS() isn't there to catch it. Again,
    there is no evidence such "correction" is beneficial.

    bloat-o-meter result:
    add/remove: 0/0 grow/shrink: 4/62 up/down: 309/-2851 (-2542)

    Signed-off-by: Yu Zhao <yuzhao@google.com>
    ---
    include/linux/page-flags.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
    index db914477057b..1995208a3763 100644
    --- a/include/linux/page-flags.h
    +++ b/include/linux/page-flags.h
    @@ -248,7 +248,7 @@ static inline void page_init_poison(struct page *page, size_t size)
    PF_POISONED_CHECK(page); })
    #define PF_NO_TAIL(page, enforce) ({ \
    VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page); \
    - PF_POISONED_CHECK(compound_head(page)); })
    + PF_POISONED_CHECK((enforce) ? (page) : compound_head(page)); })
    #define PF_NO_COMPOUND(page, enforce) ({ \
    VM_BUG_ON_PGFLAGS(enforce && PageCompound(page), page); \
    PF_POISONED_CHECK(page); })
    --
    2.30.1.766.gb4fecdf3b7-goog
    \
     
     \ /
      Last update: 2021-02-26 10:23    [W:2.631 / U:1.292 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site