Re: [RFC 1/2] arm: cacheflush syscall: process only pages that are in the memory

[Inki Dae]

Hi Russell,

2018ë 01ì 27ì 06:39ì Russell King - ARM Linux ì(ê) ì ê:
> On Fri, Jan 26, 2018 at 02:30:47PM +0100, Marek Szyprowski wrote:
>> 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").
> 
> No, you're not reading the previous code state correctly.  Take a closer
> look at that commit.
> 
> find_vma() requires that mmap_sem is held across the call as the VMA
> list is not stable without that semaphore held.  However, more
> importantly, notice that it drops the semaphore _before_ calling the
> cache flushing function (__do_cache_op()).
> 
> The point is that if __do_cache_op() faults, it will enter
> do_page_fault(), which will try to take the mmap_sem again, causing
> a deadlock.

I'm not sure but seems this patch tries to do cache-flush only in-memory pages.
So I think the page fault wouldn't happen becasue flush_cache_user_range function returns always 0.

Thanks,
Inki Dae

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