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SubjectRe: [RFC 1/2] arm: cacheflush syscall: process only pages that are in the memory
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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(&current->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(&current->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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