Messages in this thread | | | From | Blaisorblade <> | Subject | [2.6.15-rc1+ regression] do_file_page bug introduced in recent rework | Date | Fri, 2 Dec 2005 01:11:56 +0100 |
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I recently found a bug introduced in your commit 65500d234e74fc4e8f18e1a429bc24e51e75de4a, i.e. between 2.6.14 and 2.6.15-rc1, about do_file_page changes wrt remap_file_pages and MAP_POPULATE.
Quoting from the changelog (which is wrong):
do_file_page's fallback to do_no_page dates from a time when we were testing pte_file by using it wherever possible: currently it's peculiar to nonlinear vmas, so just check that. BUG_ON if not? Better not, it's probably page table corruption, so just show the pte: hmm, there's a pte_ERROR macro, let's use that for do_wp_page's invalid pfn too.
This is false:
do_mmap_pgoff: if (flags & MAP_POPULATE) { up_write(&mm->mmap_sem); sys_remap_file_pages(addr, len, 0, pgoff, flags & MAP_NONBLOCK); down_write(&mm->mmap_sem); }
So, with MAP_POPULATE|MAP_NONBLOCK passed, you can get a linear PAGE_FILE pte in a !VM_NONLINEAR vma.
That PTE is very useless since it doesn't add any information, I know that, so avoiding that possible installation is a possible fix, but for now it's simpler to change the test in do_file_page(). Btw, in fact I discovered this bug while I was implementing this optimization (working again on remap_file_pages() patches of this summer).
Indeed, the condition to test (and to possibly BUG_ON/pte_ERROR) is that ->populate must exist for the sys_remap_file_pages call to work. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894) http://www.user-mode-linux.org/~blaisorblade
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