M. Koehrer wrote:
> Hi Philippe,
>
> I agree. To fix the root cause is actually the very best to do!
> This eases the life of users and developers.
>
> Regards
>
> Mathias
Hi Mathias,
here comes a workaround for the COW issue on Linux 2.6.19. The patch
relies on a new VM_NOCOW flag which should be set for real-time
application if you use Xenomai trunk.
It would be nice if you could test it.
Thanks in advance.
--
Gilles Chanteperdrix
diff -x '*~' -Naurdp linux-2.6.19/include/linux/mm.h linux-2.6.19-nocow/include/linux/mm.h
--- linux-2.6.19/include/linux/mm.h 2006-11-30 11:04:08.000000000 +0100
+++ linux-2.6.19-nocow/include/linux/mm.h 2006-12-27 10:03:09.000000000 +0100
@@ -166,6 +166,7 @@ extern unsigned int kobjsize(const void
#define VM_NONLINEAR 0x00800000 /* Is non-linear (remap_file_pages) */
#define VM_MAPPED_COPY 0x01000000 /* T if mapped copy of data (nommu mmap) */
#define VM_INSERTPAGE 0x02000000 /* The vma has had "vm_insert_page()" done on it */
+#define VM_NOCOW 0x10000000 /* Disable COW mapping for the vma */
#ifndef VM_STACK_DEFAULT_FLAGS /* arch can override this */
#define VM_STACK_DEFAULT_FLAGS VM_DATA_DEFAULT_FLAGS
diff -x '*~' -Naurdp linux-2.6.19/mm/memory.c linux-2.6.19-nocow/mm/memory.c
--- linux-2.6.19/mm/memory.c 2006-11-30 11:04:14.000000000 +0100
+++ linux-2.6.19-nocow/mm/memory.c 2006-12-27 10:06:37.000000000 +0100
@@ -418,13 +418,41 @@ struct page *vm_normal_page(struct vm_ar
return pfn_to_page(pfn);
}
+static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va)
+{
+ /*
+ * If the source page was a PFN mapping, we don't have
+ * a "struct page" for it. We do a best-effort copy by
+ * just copying from the original user address. If that
+ * fails, we just zero-fill it. Live with it.
+ */
+ if (unlikely(!src)) {
+ void *kaddr = kmap_atomic(dst, KM_USER0);
+ void __user *uaddr = (void __user *)(va & PAGE_MASK);
+
+ /*
+ * This really shouldn't fail, because the page is there
+ * in the page tables. But it might just be unreadable,
+ * in which case we just give up and fill the result with
+ * zeroes.
+ */
+ if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
+ memset(kaddr, 0, PAGE_SIZE);
+ kunmap_atomic(kaddr, KM_USER0);
+ flush_dcache_page(dst);
+ return;
+
+ }
+ copy_user_highpage(dst, src, va);
+}
+
/*
* copy one vm_area from one task to the other. Assumes the page tables
* already present in the new task to be cleared in the whole range
* covered by this vma.
*/
-static inline void
+static inline int
copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pte_t *dst_pte, pte_t *src_pte, struct vm_area_struct *vma,
unsigned long addr, int *rss)
@@ -466,6 +494,25 @@ copy_one_pte(struct mm_struct *dst_mm, s
* in the parent and the child
*/
if (is_cow_mapping(vm_flags)) {
+#ifdef CONFIG_IPIPE
+ if (((vm_flags|src_mm->def_flags) & (VM_LOCKED|VM_NOCOW)) == (VM_LOCKED|VM_NOCOW)) {
+ struct page *old_page = vm_normal_page(vma, addr, pte);
+ page = alloc_page_vma(GFP_HIGHUSER, vma, addr);
+ if (!page)
+ return -ENOMEM;
+
+ cow_user_page(page, old_page, addr);
+ pte = mk_pte(page, vma->vm_page_prot);
+
+ if (vm_flags & VM_SHARED)
+ pte = pte_mkclean(pte);
+ pte = pte_mkold(pte);
+
+ page_dup_rmap(page);
+ rss[!!PageAnon(page)]++;
+ goto out_set_pte;
+ }
+#endif /* CONFIG_IPIPE */
ptep_set_wrprotect(src_mm, addr, src_pte);
pte = pte_wrprotect(pte);
}
@@ -487,6 +534,7 @@ copy_one_pte(struct mm_struct *dst_mm, s
out_set_pte:
set_pte_at(dst_mm, addr, dst_pte, pte);
+ return 0;
}
static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
@@ -524,7 +572,9 @@ again:
progress++;
continue;
}
- copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vma, addr, rss);
+ if (copy_one_pte(dst_mm, src_mm, dst_pte,
+ src_pte, vma, addr, rss))
+ return -ENOMEM;
progress += 8;
} while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
@@ -1431,34 +1481,6 @@ static inline pte_t maybe_mkwrite(pte_t
return pte;
}
-static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va)
-{
- /*
- * If the source page was a PFN mapping, we don't have
- * a "struct page" for it. We do a best-effort copy by
- * just copying from the original user address. If that
- * fails, we just zero-fill it. Live with it.
- */
- if (unlikely(!src)) {
- void *kaddr = kmap_atomic(dst, KM_USER0);
- void __user *uaddr = (void __user *)(va & PAGE_MASK);
-
- /*
- * This really shouldn't fail, because the page is there
- * in the page tables. But it might just be unreadable,
- * in which case we just give up and fill the result with
- * zeroes.
- */
- if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
- memset(kaddr, 0, PAGE_SIZE);
- kunmap_atomic(kaddr, KM_USER0);
- flush_dcache_page(dst);
- return;
-
- }
- copy_user_highpage(dst, src, va);
-}
-
/*
* This routine handles present pages, when users try to write
* to a shared page. It is done by copying the page to a new address
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