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

[Leonid Yegoshin]

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

diff --git a/arch/mips/include/asm/cacheflush.h 
b/arch/mips/include/asm/cacheflush.h
index 42e5fc682590..27b17b16a96d 100644
--- a/arch/mips/include/asm/cacheflush.h
+++ b/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.

- Leonid.

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 On Wed, 2014-12-03 at 
> 14:42 +0100, Ralf Baechle wrote:
> > Lars, normally set_pte_at() is invoked in a cache_flush_*() 
> > set_pte_at() tlb_flush_*() sequence. So I'm wondering if you're 
> > trying to fix something in set_pte_at that actually ought to be fixed 
> > in the cache_flush_*() function. I'm wondering, have you identified 
> > which cache flush function in particular was used in the sequence in 
> > your particular bug's case? Ralf 

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