On Sat, 2015-10-31 at 14:58 +0300, Stas Sergeev wrote: > 29.10.2015 01:51, Toshi Kani пишет: > > On Wed, 2015-10-28 at 13:22 -0600, Toshi Kani wrote: > > > On Wed, 2015-10-28 at 10:34 -0600, Toshi Kani wrote:
: > > > I looked at the dosemu code and was able to reproduce the issue with a > > > test program. This problem happens when mremap() to /dev/mem (or PFNMAP) > > > is called with MREMAP_FIXED. > > > > > > In this case, mremap calls move_vma(), which first calls > > > move_page_tables() to remap the translation and then calls do_munmap() to > > > remove the original mapping. Hence, when untrack_pfn() is called from > > > do_munmap(), the original map is already removed, and follow_phys() fails > > > with the !pte_present() check. > > > > > > I think there are a couple of issues: > > > - If untrack_pfn() ignores an error from follow_phys() and skips > > > free_pfn_range(), PAT continues to track the original map that is removed. > > > - untrack_pfn() calls free_pfn_range() to untrack a given free range. > > > However, rbt_memtype_erase() requires the free range match exactly to > > > the tracked range. This does not support mremap, which needs to free up > > > part of the tracked range. > > > - PAT does not track a new translation specified by mremap() with > > > MREMAP_FIXED. > > Thinking further, I think the 1st and 3rd items are non-issues. mremap > > remaps virtual address, but keeps the same cache type and pfns. So, PAT > > does not have to change the tracked pfns in this case. The 2nd item is > > still a problem, though. > Hello Toshi, thanks for your analysis. > Now as you do not seem to be preparing a fix, how > about attaching your test-case to the bug-report for > others to re-use? Or maybe you can even make it a > part of the kernel's test suit - I suppose this will help. I can work on the fix, but I did not think we needed to rush on it since this issue exists for a long time. Can we target it to 4.4-rcX and then cc to stables? As for the test, it might be tricky to make it run on any platforms since it needs to mmap/mremap a non-RAM range. It could just map a reserved range randomly, but I chose a range manually which I know is safe to map & read/write on my platform in my test. Thanks, -Toshi -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
