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

From: Leonid Yegoshin
Date: Thu Dec 04 2014 - 21:16:55 EST

(repeat mesg, first one went to wrong place)


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


@@ -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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