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Date: Wed, 15 Aug 2018 12:16:06 -0700
From: Matthew Wilcox <willy@...radead.org>
To: Yang Shi <yang.shi@...ux.alibaba.com>
Cc: mhocko@...nel.org, ldufour@...ux.vnet.ibm.com,
kirill@...temov.name, vbabka@...e.cz, akpm@...ux-foundation.org,
peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
namhyung@...nel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap_sem in
munmap
On Thu, Aug 16, 2018 at 02:49:48AM +0800, Yang Shi wrote:
> +static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
> + size_t len, struct list_head *uf)
> +{
> + unsigned long end;
> + struct vm_area_struct *start_vma, *prev, *vma;
> + int ret = 0;
> +
> + if (!addr_ok(start, len))
> + return -EINVAL;
> +
> + len = PAGE_ALIGN(len);
> +
> + end = start + len;
> +
> + /*
> + * Need write mmap_sem to split vmas and detach vmas
> + * splitting vma up-front to save PITA to clean if it is failed
> + */
> + if (down_write_killable(&mm->mmap_sem))
> + return -EINTR;
> +
> + start_vma = munmap_lookup_vma(mm, start, end);
> + if (!start_vma)
> + goto out;
> + if (IS_ERR(start_vma)) {
> + ret = PTR_ERR(start_vma);
> + goto out;
> + }
> +
> + prev = start_vma->vm_prev;
> +
> + if (unlikely(uf)) {
> + ret = userfaultfd_unmap_prep(start_vma, start, end, uf);
> + if (ret)
> + goto out;
> + }
> +
> + /*
> + * Unmapping vmas, which have:
> + * VM_HUGETLB or
> + * VM_PFNMAP or
> + * uprobes
> + * need get done with write mmap_sem held since they may update
> + * vm_flags. Deal with such mappings with regular do_munmap() call.
> + */
> + for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) {
> + if ((vma->vm_file &&
> + has_uprobes(vma, vma->vm_start, vma->vm_end)) ||
> + (vma->vm_flags & (VM_HUGETLB | VM_PFNMAP)))
> + goto regular_path;
but ... that's going to redo all the work you already did! Why not just this:
(not even compiled, and I can see a good opportunity for combining the
VM_LOCKED loop with the has_uprobes loop)
diff --git a/mm/mmap.c b/mm/mmap.c
index de699523c0b7..8d121db36efc 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2803,6 +2803,8 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
{
unsigned long end;
struct vm_area_struct *vma, *prev, *last;
+ int res = 0;
+ bool downgrade = false;
if ((offset_in_page(start)) || start > TASK_SIZE || len > TASK_SIZE-start)
return -EINVAL;
@@ -2811,17 +2813,20 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
if (len == 0)
return -EINVAL;
+ if (down_write_killable(&mm->mmap_sem))
+ return -EINTR;
+
/* Find the first overlapping VMA */
vma = find_vma(mm, start);
if (!vma)
- return 0;
+ goto unlock;
prev = vma->vm_prev;
- /* we have start < vma->vm_end */
+ /* we have start < vma->vm_end */
/* if it doesn't overlap, we have nothing.. */
end = start + len;
if (vma->vm_start >= end)
- return 0;
+ goto unlock;
/*
* If we need to split any vma, do it now to save pain later.
@@ -2831,28 +2836,27 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
* places tmp vma above, and higher split_vma places tmp vma below.
*/
if (start > vma->vm_start) {
- int error;
-
/*
* Make sure that map_count on return from munmap() will
* not exceed its limit; but let map_count go just above
* its limit temporarily, to help free resources as expected.
*/
+ res = -ENOMEM
if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
- return -ENOMEM;
+ goto unlock;
- error = __split_vma(mm, vma, start, 0);
- if (error)
- return error;
+ res = __split_vma(mm, vma, start, 0);
+ if (res)
+ goto unlock;
prev = vma;
}
/* Does it split the last one? */
last = find_vma(mm, end);
if (last && end > last->vm_start) {
- int error = __split_vma(mm, last, end, 1);
- if (error)
- return error;
+ res = __split_vma(mm, last, end, 1);
+ if (res)
+ goto unlock;
}
vma = prev ? prev->vm_next : mm->mmap;
@@ -2866,9 +2870,19 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
* split, despite we could. This is unlikely enough
* failure that it's not worth optimizing it for.
*/
- int error = userfaultfd_unmap_prep(vma, start, end, uf);
- if (error)
- return error;
+ result = userfaultfd_unmap_prep(vma, start, end, uf);
+ if (result)
+ goto unlock;
+ }
+
+ downgrade = true;
+
+ for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) {
+ if (vma->vm_file &&
+ has_uprobes(vma, vma->vm_start, vma->vm_end)) {
+ downgrade = false;
+ break;
+ }
}
/*
@@ -2885,6 +2899,9 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
}
}
+ if (downgrade)
+ downgrade_write(&mm->mmap_sem);
+
/*
* Remove the vma's, and unmap the actual pages
*/
@@ -2896,7 +2913,14 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
/* Fix up all other VM information */
remove_vma_list(mm, vma);
- return 0;
+ res = 0;
+unlock:
+ if (downgrade) {
+ up_read(&mm->mmap_sem);
+ } else {
+ up_write(&mm->mmap_sem);
+ }
+ return res;
}
int vm_munmap(unsigned long start, size_t len)
@@ -2905,11 +2929,7 @@ int vm_munmap(unsigned long start, size_t len)
struct mm_struct *mm = current->mm;
LIST_HEAD(uf);
- if (down_write_killable(&mm->mmap_sem))
- return -EINTR;
-
ret = do_munmap(mm, start, len, &uf);
- up_write(&mm->mmap_sem);
userfaultfd_unmap_complete(mm, &uf);
return ret;
}
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