On Wed, 23 Jan 2008, Anton Salikhmetov wrote:
> +
> + if (pte_dirty(*pte) && pte_write(*pte)) {
Not correct.
You still need to check "pte_present()" before you can test any other
bits. For a non-present pte, none of the other bits are defined, and for
all we know there might be architectures out there that require them to
be non-dirty.
As it is, you just possibly randomly corrupted the pte.
Yeah, on all architectures I know of, it the pte is clear, neither of
those tests will trigger, so it just happens to work, but it's still
wrong. And for a MAP_SHARED mapping, it should be either clear or valid,
although I can imagine that we might do swap-cache entries for tmpfs or
something (in which case trying to clear the write-enable bit would
corrupt the swap entry!).
So the bug might be hard or even impossible to trigger in practice, but
it's still wrong.
I realize that "page_mkclean_one()" doesn't do this very obviously, but
it's actually there (it's just hidden in page_check_address()).
Quite frankly, at this point I'm getting *very* tired of this series.
Especially since you ignored me when I suggested you just revert the
commit that removed the page table walking - and instead send in a buggy
patch.
Yes, the VM is hard. I agree. It's nasty. But exactly because it's nasty
and subtle and horrid, I'm also very anal about it, and I get really
nervous when somebody touches it without (a) knowing all the rules
intimately and (b) listening to people who do.
So here's even a patch to get you started. Do this:
git revert 204ec841fbea3e5138168edbc3a76d46747cc987
and then use this appended patch on top of that as a starting point for
something that compiles and *possibly* works.
And no, I do *not* guarantee that this is right either! I have not tested
it or thought about it a lot, and S390 tends to be odd about some of these
things. In particular, I actually suspect that we should possibly do this
the same way we do
ptep_clear_flush_young()
except we would do "ptep_clear_flush_wrprotect()". So even though this is
a revert plus a simple patch to make it compile again (we've changed how
we do dirty bits), I think a patch like this needs testing and other
people like Nick and Peter to ack it.
Nick? Peter? Testing? Other comments?
Linus
---
mm/msync.c | 9 ++++++---
1 files changed, 6 insertions(+), 3 deletions(-)
diff --git a/mm/msync.c b/mm/msync.c
index a30487f..9b0af8f 100644
--- a/mm/msync.c
+++ b/mm/msync.c
@@ -32,6 +32,7 @@ static unsigned long msync_pte_range(struct vm_area_struct
*vma, pmd_t *pmd,
again:
pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
do {
+ pte_t entry;
struct page *page;
if (progress >= 64) {
@@ -47,9 +48,11 @@ again:
page = vm_normal_page(vma, addr, *pte);
if (!page)
continue;
- if (ptep_clear_flush_dirty(vma, addr, pte) ||
- page_test_and_clear_dirty(page))
- ret += set_page_dirty(page);
+ entry = ptep_clear_flush(vma, addr, pte);
+ entry = pte_wrprotect(entry);
+ set_pte_at(mm, address, pte, entry);
+
+ ret += 1;
progress += 3;
} while (pte++, addr += PAGE_SIZE, addr != end);
pte_unmap_unlock(pte - 1, ptl);
--
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