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Subject[PATCH 4/5] uprobes: cleanup and document write_opcode()->lock_page(old_page)
The comment above write_opcode()->lock_page(old_page) tells about
the race with do_wp_page(). I don't really understand which exactly
race it means, but afaics this lock_page() was not enough to close
all races with do_wp_page().

Anyway, since 77fc4af1 this code is always called with ->mmap_sem
hold for writing so we can forget about do_wp_page().

However, we can't simply remove this lock_page(), and the only
(afaics) reason is __replace_page()->try_to_free_swap().

Nothing in write_opcode() needs it, move it into __replace_page()
and fix the comment.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/events/uprobes.c | 27 ++++++++++++++-------------
1 files changed, 14 insertions(+), 13 deletions(-)
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 16a3b69..442064d 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -138,10 +138,15 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
struct mm_struct *mm = vma->vm_mm;
spinlock_t *ptl;
pte_t *ptep;
+ int err;

+ /* freeze PageSwapCache() for try_to_free_swap() below */
+ lock_page(page);
+
+ err = -EAGAIN;
ptep = page_check_address(page, mm, addr, &ptl, 0);
if (!ptep)
- return -EAGAIN;
+ goto unlock;

get_page(kpage);
page_add_new_anon_rmap(kpage, vma, addr);
@@ -161,7 +166,10 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
put_page(page);
pte_unmap_unlock(ptep, ptl);

- return 0;
+ err = 0;
+ unlock:
+ unlock_page(page);
+ return err;
}

/**
@@ -215,15 +223,10 @@ retry:
ret = -ENOMEM;
new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vaddr);
if (!new_page)
- goto put_out;
+ goto put_old;

__SetPageUptodate(new_page);

- /*
- * lock page will serialize against do_wp_page()'s
- * PageAnon() handling
- */
- lock_page(old_page);
/* copy the page now that we've got it stable */
vaddr_old = kmap_atomic(old_page);
vaddr_new = kmap_atomic(new_page);
@@ -236,15 +239,13 @@ retry:

ret = anon_vma_prepare(vma);
if (ret)
- goto unlock_out;
+ goto put_new;

ret = __replace_page(vma, vaddr, old_page, new_page);

-unlock_out:
- unlock_page(old_page);
+put_new:
page_cache_release(new_page);
-
-put_out:
+put_old:
put_page(old_page);

if (unlikely(ret == -EAGAIN))
--
1.5.5.1



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