Re: [PATCH 1/5] mm/madvise: remove the visitor pattern and thread anon_vma state

From: Lance Yang
Date: Thu Jun 19 2025 - 22:43:53 EST



On 2025/6/20 09:32, Zi Yan wrote:
On 19 Jun 2025, at 16:26, Lorenzo Stoakes wrote:

Now we have the madvise_behavior helper struct we no longer need to mess
around with void* pointers in order to propagate anon_vma_name, and this
means we can get rid of the confusing and inconsistent visitor pattern
implementation in madvise_vma_anon_name().

This means we now have a single state object that threads through most of
madvise()'s logic and a single code path which executes the majority of
madvise() behaviour (we maintain separate logic for failure injection and
memory population for the time being).

Note that users cannot inadvertently cause this behaviour to occur, as
madvise_behavior_valid() would reject it.

Doing this results in a can_modify_vma_madv() check for anonymous VMA name
changes, however this will cause no issues as this operation is not
prohibited.

We can also then reuse more code and drop the redundant
madvise_vma_anon_name() function altogether.

Additionally separate out behaviours that update VMAs from those that do
not.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
mm/madvise.c | 167 +++++++++++++++++++++++++++------------------------
1 file changed, 90 insertions(+), 77 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 070132f9842b..9dd935d64692 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -37,6 +37,9 @@
#include "internal.h"
#include "swap.h"

+#define __MADV_SET_ANON_VMA_NAME (-1)
+#define __MADV_CLEAR_ANON_VMA_NAME (-2)
+

These are stored in madvise_behavior.behavior field and used
internally. At least you could add a comment in mman-common.h
about this use, in case someone uses these values for MADV_*.
Yes, these are large values that are very unlikely to be used,
but who knows whether one will use them. :)

/*
* Maximum number of attempts we make to install guard pages before we give up
* and return -ERESTARTNOINTR to have userspace try again.
@@ -59,9 +62,13 @@ struct madvise_behavior {
int behavior;
struct mmu_gather *tlb;
enum madvise_lock_mode lock_mode;
+ struct anon_vma_name *anon_name;
};

#ifdef CONFIG_ANON_VMA_NAME
+static int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
+ unsigned long end, struct madvise_behavior *madv_behavior);
+
struct anon_vma_name *anon_vma_name_alloc(const char *name)
{
struct anon_vma_name *anon_name;
@@ -112,6 +119,48 @@ static int replace_anon_vma_name(struct vm_area_struct *vma,

return 0;
}
+
+int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
+ unsigned long len_in, struct anon_vma_name *anon_name)
+{
+ unsigned long end;
+ unsigned long len;
+ struct madvise_behavior madv_behavior = {
+ .lock_mode = MADVISE_MMAP_WRITE_LOCK,
+ .anon_name = anon_name,
+ };
+
+ if (start & ~PAGE_MASK)
+ return -EINVAL;
+ len = (len_in + ~PAGE_MASK) & PAGE_MASK;
+
+ /* Check to see whether len was rounded up from small -ve to zero */
+ if (len_in && !len)
+ return -EINVAL;
+
+ end = start + len;
+ if (end < start)
+ return -EINVAL;
+
+ if (end == start)
+ return 0;
+
+ madv_behavior.behavior =
+ anon_name ? __MADV_SET_ANON_VMA_NAME : __MADV_CLEAR_ANON_VMA_NAME;

How are __MADV_SET_ANON_VMA_NAME and __MADV_CLEAR_ANON_VMA_NAME used?
It seems to me that madvise_vma_behavior() treats them the same.
Why does anon_name is NULL or not make a difference?

+
+ return madvise_walk_vmas(mm, start, end, &madv_behavior);
+}
+
+static bool is_anon_vma_name(int behavior)

Maybe update_anon_vma_name()? Otherwise the function reads like
the behavior can be anon_vma_name.

+{
+ switch (behavior) {
+ case __MADV_SET_ANON_VMA_NAME:
+ case __MADV_CLEAR_ANON_VMA_NAME:
+ return true;
+ default:
+ return false;
+ }
+}
#else /* CONFIG_ANON_VMA_NAME */
static int replace_anon_vma_name(struct vm_area_struct *vma,
struct anon_vma_name *anon_name)
@@ -121,6 +170,11 @@ static int replace_anon_vma_name(struct vm_area_struct *vma,

return 0;
}
+
+static bool is_anon_vma_name(int behavior)
+{
+ return false;
+}
#endif /* CONFIG_ANON_VMA_NAME */
/*
* Update the vm_flags on region of a vma, splitting it or merging it as
@@ -1252,13 +1306,12 @@ static long madvise_guard_remove(struct vm_area_struct *vma,
static int madvise_vma_behavior(struct vm_area_struct *vma,
struct vm_area_struct **prev,
unsigned long start, unsigned long end,
- void *behavior_arg)
+ struct madvise_behavior *madv_behavior)
{
- struct madvise_behavior *arg = behavior_arg;
- int behavior = arg->behavior;
- int error;
- struct anon_vma_name *anon_name;
+ int behavior = madv_behavior->behavior;
+ struct anon_vma_name *anon_name = madv_behavior->anon_name;
vm_flags_t new_flags = vma->vm_flags;
+ int error;

if (unlikely(!can_modify_vma_madv(vma, behavior)))
return -EPERM;
@@ -1275,7 +1328,17 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
case MADV_FREE:
case MADV_DONTNEED:
case MADV_DONTNEED_LOCKED:
- return madvise_dontneed_free(vma, prev, start, end, arg);
+ return madvise_dontneed_free(vma, prev, start, end,
+ madv_behavior);
+ case MADV_COLLAPSE:
+ return madvise_collapse(vma, prev, start, end);
+ case MADV_GUARD_INSTALL:
+ return madvise_guard_install(vma, prev, start, end);
+ case MADV_GUARD_REMOVE:
+ return madvise_guard_remove(vma, prev, start, end);
+
+ /* The below behaviours update VMAs via madvise_update_vma(). */
+

Great comment and code move!

case MADV_NORMAL:
new_flags = new_flags & ~VM_RAND_READ & ~VM_SEQ_READ;
break;
@@ -1325,21 +1388,25 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
if (error)
goto out;
break;
- case MADV_COLLAPSE:
- return madvise_collapse(vma, prev, start, end);
- case MADV_GUARD_INSTALL:
- return madvise_guard_install(vma, prev, start, end);
- case MADV_GUARD_REMOVE:
- return madvise_guard_remove(vma, prev, start, end);
+ case __MADV_SET_ANON_VMA_NAME:
+ case __MADV_CLEAR_ANON_VMA_NAME:


Would this be clearer and more closely describe the code's logic?

/* Reject naming for regular file-backed mappings. */


+ /* Only anonymous mappings can be named */
+ if (vma->vm_file && !vma_is_anon_shmem(vma))
+ return -EBADF;
+ break;
}


Thanks,
Lance


__MADV_SET_ANON_VMA_NAME and __MADV_CLEAR_ANON_VMA_NAME are
used here the code below. I do not see the functional difference
of them. I understand a NULL anon_name means clear the name,
but it is also just set the name to NULL.


/* We cannot provide prev in this lock mode. */
- VM_WARN_ON_ONCE(arg->lock_mode == MADVISE_VMA_READ_LOCK);
- anon_name = anon_vma_name(vma);
- anon_vma_name_get(anon_name);
+ VM_WARN_ON_ONCE(madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK);
+
+ if (!is_anon_vma_name(behavior)) {
+ anon_name = anon_vma_name(vma);
+ anon_vma_name_get(anon_name);
+ }
error = madvise_update_vma(vma, prev, start, end, new_flags,
anon_name);
- anon_vma_name_put(anon_name);
+ if (!is_anon_vma_name(behavior))
+ anon_vma_name_put(anon_name);

Otherwise, the rest looks good to me.

--
Best Regards,
Yan, Zi