Re: [PATCH] Revert "MIPS: Remove race window in page fault handling"

[Lars Persson]

Hi

Our setup includes both a non-DMA block device and a compressing
file-system (UBIFS). A flush_dcache_page() is issued by UBIFS so your
patch fixes another problem that we do not hit.

The stack trace is not available now. Do we need it for any further
analysis ? I think the mechanism of the race window is understood and it
depends on the __flush_dcache_page() deciding that the flush should be
postponed.

- Lars

On Fri, 2014-12-05 at 03:16 +0100, Leonid Yegoshin wrote:
> (repeat mesg, first one went to wrong place)
> 
> Lars,
> 
> Do you have a stack trace or so then you found the second VPE between 
> set_pte_at and update_mmu_cache?
> It would be interesting how it happens - generally, to get a consistent 
> SIGILL in applications due to misbehaviour of memory subsystem, the bug 
> in FS is not enough.
> 
> Hold on - do you use non-DMA file system?
> If so, I advice you to try this simple patch:
> 
>      Author: Leonid Yegoshin <yegoshin@xxxxxxxx>
>      Date:   Tue Apr 2 14:20:37 2013 -0700
> 
>      MIPS: (opt) Fix of reading I-pages from non-DMA FS devices for ID 
> cache separation
> 
>      This optional fix provides a D-cache flush for instruction code 
> pages on
>      page faults. In case of non-DMA block device a driver doesn't know 
> that it
>      reads I-page and doesn't flush D-cache generally on systems without
>      cache aliasing. And that takes toll during page fault of 
> instruction pages.
> 
>      It is not a perfect fix, it should be considered as a temporary fix.
>      The permanent fix would track page origin in page cache and flushes 
> D-cache
>      during reception of page from driver only but not at each page fault.
>      It is not done yet.
> 
>      Change-Id: I43f5943d6ce0509729179615f6b81e77803a34ac
>      Author: Leonid Yegoshin <yegoshin@xxxxxxxx>
>      Signed-off-by: Leonid Yegoshin <yegoshin@xxxxxxxx>(imported from 
> commit 6ebd22eb7a3d9873582ebe990a77094f971652ee)(imported from commit 
> 0caf3b4a1eebb64572e81e4df6fdb3abf12c70
> 
> arch/mips/include/asm/cacheflush.h:
> 
>     @@ -61,6 +61,9 @@ static inline void flush_anon_page(struct 
> vm_area_struct *vma,
>      static inline void flush_icache_page(struct vm_area_struct *vma,
>             struct page *page)
>      {
>     +       if (cpu_has_dc_aliases ||
>     +           ((vma->vm_flags & VM_EXEC) && !cpu_has_ic_fills_f_dc))
>     +               __flush_dcache_page(page);
>      }
> 
>      extern void (*flush_icache_range)(unsigned long start, unsigned 
> long end);
> 
> 
> It fixed crash problems with non-DMA FS in a couple of our customers. 
> Without it the non-DMA root FS crashes are catastrophic in aliasing 
> systems but it is still a problem for I-cache too but much rare.
> 
> Unfortunately, it is also a performance hit, however is less than run a 
> page cache flush at each PTE setup. On 12/03/2014 06:03 AM, Lars Persson 
> wrote:
> > It is the flush_dcache_page() that was called from the file-system
> > reading the page contents into memory.
> >
> > - Lars
> >
> >
> 



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