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

[Marek Szyprowski]

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.szyprow...@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