Messages in this thread | | | Date | Wed, 31 Jan 2018 15:03:23 +0900 | From | Inki Dae <> | Subject | Re: [RFC 1/2] arm: cacheflush syscall: process only pages that are in the memory |
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Hi Russell,
2018년 01월 27일 06:39에 Russell King - ARM Linux 이(가) 쓴 글: > 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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