Re: [RFC v6 PATCH 1/2] mm: refactor do_munmap() to extract the common part
From: Yang Shi
Date: Fri Aug 03 2018 - 16:47:41 EST
On 8/3/18 1:53 AM, Michal Hocko wrote:
On Fri 27-07-18 02:10:13, Yang Shi wrote:
Introduces three new helper functions:
* munmap_addr_sanity()
* munmap_lookup_vma()
* munmap_mlock_vma()
They will be used by do_munmap() and the new do_munmap with zapping
large mapping early in the later patch.
There is no functional change, just code refactor.
Reviewed-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 120 ++++++++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 82 insertions(+), 38 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index d1eb87e..2504094 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2686,34 +2686,44 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
return __split_vma(mm, vma, addr, new_below);
}
-/* Munmap is split into 2 main parts -- this part which finds
- * what needs doing, and the areas themselves, which do the
- * work. This now handles partial unmappings.
- * Jeremy Fitzhardinge <jeremy@xxxxxxxx>
- */
-int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
- struct list_head *uf)
+static inline bool munmap_addr_sanity(unsigned long start, size_t len)
munmap_check_addr? Btw. why does this need to have munmap prefix at all?
This is a general address space check.
Just because I extracted this from do_munmap, no special consideration.
It is definitely ok to use another name.
{
- unsigned long end;
- struct vm_area_struct *vma, *prev, *last;
-
if ((offset_in_page(start)) || start > TASK_SIZE || len > TASK_SIZE-start)
- return -EINVAL;
+ return false;
- len = PAGE_ALIGN(len);
- if (len == 0)
- return -EINVAL;
+ if (PAGE_ALIGN(len) == 0)
+ return false;
+
+ return true;
+}
+
+/*
+ * munmap_lookup_vma: find the first overlap vma and split overlap vmas.
+ * @mm: mm_struct
+ * @vma: the first overlapping vma
+ * @prev: vma's prev
+ * @start: start address
+ * @end: end address
This really doesn't help me to understand how to use the function.
Why do we need both prev and vma etc...
prev will be used by unmap_region later.
+ *
+ * returns 1 if successful, 0 or errno otherwise
This is a really weird calling convention. So what does 0 tell? /me
checks the code. Ohh, it is nothing to do. Why cannot you simply return
the vma. NULL implies nothing to do, ERR_PTR on error.
A couple of reasons why it is implemented as so:
ÂÂÂ * do_munmap returns 0 for both success and no suitable vma
ÂÂÂ * Since prev is needed by finding the start vma, and prev will be
used by unmap_region later too, so I just thought it would look clean to
have one function to return both start vma and prev. In this way, we can
share as much as possible common code.
ÂÂÂ * In this way, we just need return 0, 1 or error no just as same as
what do_munmap does currently. Then we know what is failure case exactly
to just bail out right away.
Actually, I tried the same approach as you suggested, but it had two
problems:
ÂÂÂ * If it returns the start vma, we have to re-find its prev later,
but the prev has been found during finding start vma. And, duplicate the
code in do_munmap_zap_rlock. It sounds not that ideal.
ÂÂÂ * If it returns prev, it might be null (start vma is the first
vma). We can't tell if null is a failure or success case
+ */
+static int munmap_lookup_vma(struct mm_struct *mm, struct vm_area_struct **vma,
+ struct vm_area_struct **prev, unsigned long start,
+ unsigned long end)
+{
+ struct vm_area_struct *tmp, *last;
/* Find the first overlapping VMA */
- vma = find_vma(mm, start);
- if (!vma)
+ tmp = find_vma(mm, start);
+ if (!tmp)
return 0;
- prev = vma->vm_prev;
- /* we have start < vma->vm_end */
+
+ *prev = tmp->vm_prev;
Why do you set prev here. We might "fail" with 0 right after this
No special reason, just copied from do_munmap. Yes, it is ideal to have
prev set here. It can be moved further down.
+
+ /* we have start < vma->vm_end */
/* if it doesn't overlap, we have nothing.. */
- end = start + len;
- if (vma->vm_start >= end)
+ if (tmp->vm_start >= end)
return 0;
/*
@@ -2723,7 +2733,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
* unmapped vm_area_struct will remain in use: so lower split_vma
* places tmp vma above, and higher split_vma places tmp vma below.
*/
- if (start > vma->vm_start) {
+ if (start > tmp->vm_start) {
int error;
/*
@@ -2731,13 +2741,14 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
* not exceed its limit; but let map_count go just above
* its limit temporarily, to help free resources as expected.
*/
- if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
+ if (end < tmp->vm_end &&
+ mm->map_count > sysctl_max_map_count)
return -ENOMEM;
- error = __split_vma(mm, vma, start, 0);
+ error = __split_vma(mm, tmp, start, 0);
if (error)
return error;
- prev = vma;
+ *prev = tmp;
}
/* Does it split the last one? */
@@ -2747,7 +2758,48 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
if (error)
return error;
}
- vma = prev ? prev->vm_next : mm->mmap;
+
+ *vma = *prev ? (*prev)->vm_next : mm->mmap;
+
+ return 1;
+}
the patch would be much more easier to read if you didn't do vma->tmp
renaming.
Yes, I should used another name for the "vma" argument.
Thanks,
Yang