Messages in this thread | | | Subject | Re: [RFC 1/2] arm: cacheflush syscall: process only pages that are in the memory | From | Marek Szyprowski <> | Date | Fri, 26 Jan 2018 14:30:47 +0100 |
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Hi Russell,
On 2018-01-26 12:32, Russell King - ARM Linux wrote: > On Fri, Jan 26, 2018 at 12:14:40PM +0100, Marek Szyprowski wrote: >> glibc in calls cacheflush syscall on the whole textrels section of the >> relocated binaries. However, relocation usually doesn't touch all pages >> of that section, so not all of them are read to memory when calling this >> syscall. However flush_cache_user_range() function will unconditionally >> touch all pages from the provided range, resulting additional overhead >> related to reading all clean pages. Optimize this by calling >> flush_cache_user_range() only on the pages that are already in the >> memory. > What ensures that another CPU doesn't remove a page while we're > flushing it? That will trigger a data abort, which will want to > take the mmap_sem, causing a deadlock.
I thought that taking mmap_sem will prevent pages from being removed. mmap_sem has been already taken in the previous implementation of that syscall, until code simplification done by commit 97c72d89ce0e ("ARM: cacheflush: don't bother rounding to nearest vma").
>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com> >> --- >> arch/arm/kernel/traps.c | 25 +++++++++++++++++++------ >> 1 file changed, 19 insertions(+), 6 deletions(-) >> >> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c >> index 5e3633c24e63..a5ec262ab30e 100644 >> --- a/arch/arm/kernel/traps.c >> +++ b/arch/arm/kernel/traps.c >> @@ -564,23 +564,36 @@ static int bad_syscall(int n, struct pt_regs *regs) >> static inline int >> __do_cache_op(unsigned long start, unsigned long end) >> { >> - int ret; >> + struct vm_area_struct *vma = NULL; >> + int ret = 0; >> >> + down_read(¤t->mm->mmap_sem); >> do { >> unsigned long chunk = min(PAGE_SIZE, end - start); >> >> + if (!vma || vma->vm_end <= start) { >> + vma = find_vma(current->mm, start); >> + if (!vma) { >> + ret = -EFAULT; >> + goto done; >> + } >> + } >> + >> if (fatal_signal_pending(current)) >> return 0; >> >> - ret = flush_cache_user_range(start, start + chunk); >> - if (ret) >> - return ret; >> + if (follow_page(vma, start, 0)) { >> + ret = flush_cache_user_range(start, start + chunk); >> + if (ret) >> + goto done; >> + } >> >> cond_resched(); >> start += chunk; >> } while (start < end); >> - >> - return 0; >> +done: >> + up_read(¤t->mm->mmap_sem); >> + return ret; >> } >> >> static inline int >> -- >> 2.15.0 >>
Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
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