Re: [PATCH v3 2/7] mm: Add a walk_page_mapping() function to the pagewalk code

From: Thomas HellstrÃm (VMware)
Date: Thu Oct 03 2019 - 07:32:53 EST

On 10/3/19 1:17 PM, Kirill A. Shutemov wrote:
On Wed, Oct 02, 2019 at 03:47:25PM +0200, Thomas HellstrÃm (VMware) wrote:
From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>

For users that want to travers all page table entries pointing into a
region of a struct address_space mapping, introduce a walk_page_mapping()
function.

The walk_page_mapping() function will be initially be used for dirty-
tracking in virtual graphics drivers.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
---
include/linux/pagewalk.h | 9 ++++
mm/pagewalk.c | 99 +++++++++++++++++++++++++++++++++++++++-
2 files changed, 107 insertions(+), 1 deletion(-)

diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index bddd9759bab9..6ec82e92c87f 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -24,6 +24,9 @@ struct mm_walk;
* "do page table walk over the current vma", returning
* a negative value means "abort current page table walk
* right now" and returning 1 means "skip the current vma"
+ * @pre_vma: if set, called before starting walk on a non-null vma.
+ * @post_vma: if set, called after a walk on a non-null vma, provided
+ * that @pre_vma and the vma walk succeeded.
*/
struct mm_walk_ops {
int (*pud_entry)(pud_t *pud, unsigned long addr,
@@ -39,6 +42,9 @@ struct mm_walk_ops {
struct mm_walk *walk);
int (*test_walk)(unsigned long addr, unsigned long next,
struct mm_walk *walk);
+ int (*pre_vma)(unsigned long start, unsigned long end,
+ struct mm_walk *walk);
+ void (*post_vma)(struct mm_walk *walk);
};
/**
@@ -62,5 +68,8 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
void *private);
int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
void *private);
+int walk_page_mapping(struct address_space *mapping, pgoff_t first_index,
+ pgoff_t nr, const struct mm_walk_ops *ops,
+ void *private);
#endif /* _LINUX_PAGEWALK_H */
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index d48c2a986ea3..658d1e5ec428 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -253,13 +253,23 @@ static int __walk_page_range(unsigned long start, unsigned long end,
{
int err = 0;
struct vm_area_struct *vma = walk->vma;
+ const struct mm_walk_ops *ops = walk->ops;
+
+ if (vma && ops->pre_vma) {
+ err = ops->pre_vma(start, end, walk);
+ if (err)
+ return err;
+ }
if (vma && is_vm_hugetlb_page(vma)) {
- if (walk->ops->hugetlb_entry)
+ if (ops->hugetlb_entry)
err = walk_hugetlb_range(start, end, walk);
} else
err = walk_pgd_range(start, end, walk);
+ if (vma && ops->post_vma)
+ ops->post_vma(walk);
+
return err;
}
@@ -285,11 +295,17 @@ static int __walk_page_range(unsigned long start, unsigned long end,
* - <0 : failed to handle the current entry, and return to the caller
* with error code.
*
+ *
* Before starting to walk page table, some callers want to check whether
* they really want to walk over the current vma, typically by checking
* its vm_flags. walk_page_test() and @ops->test_walk() are used for this
* purpose.
*
+ * If operations need to be staged before and committed after a vma is walked,
+ * there are two callbacks, pre_vma() and post_vma(). Note that post_vma(),
+ * since it is intended to handle commit-type operations, can't return any
+ * errors.
+ *
* struct mm_walk keeps current values of some common data like vma and pmd,
* which are useful for the access from callbacks. If you want to pass some
* caller-specific data to callbacks, @private should be helpful.
@@ -376,3 +392,84 @@ int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
return err;
return __walk_page_range(vma->vm_start, vma->vm_end, &walk);
}
+
+/**
+ * walk_page_mapping - walk all memory areas mapped into a struct address_space.
+ * @mapping: Pointer to the struct address_space
+ * @first_index: First page offset in the address_space
+ * @nr: Number of incremental page offsets to cover
+ * @ops: operation to call during the walk
+ * @private: private data for callbacks' usage
+ *
+ * This function walks all memory areas mapped into a struct address_space.
+ * The walk is limited to only the given page-size index range, but if
+ * the index boundaries cross a huge page-table entry, that entry will be
+ * included.
+ *
+ * Also see walk_page_range() for additional information.
+ *
+ * Locking:
+ * This function can't require that the struct mm_struct::mmap_sem is held,
+ * since @mapping may be mapped by multiple processes. Instead
+ * @mapping->i_mmap_rwsem must be held. This might have implications in the
+ * callbacks, and it's up tho the caller to ensure that the
+ * struct mm_struct::mmap_sem is not needed.
+ *
+ * Also this means that a caller can't rely on the struct
+ * vm_area_struct::vm_flags to be constant across a call,
+ * except for immutable flags. Callers requiring this shouldn't use
+ * this function.
+ *
+ * If @mapping allows faulting of huge pmds and puds, it is desirable
+ * that its huge_fault() handler blocks while this function is running on
+ * @mapping. Otherwise a race may occur where the huge entry is split when
+ * it was intended to be handled in a huge entry callback. This requires an
+ * external lock, for example that @mapping->i_mmap_rwsem is held in
+ * write mode in the huge_fault() handlers.
Em. No. We have ptl for this. It's the only lock required (plus mmap_sem
on read) to split PMD entry into PTE table. And it can happen not only
from fault path.

If you care about splitting compound page under you, take a pin or lock a
page. It will block split_huge_page().

Suggestion to block fault path is not viable (and it will not happen
magically just because of this comment).

I was specifically thinking of this:

https://elixir.bootlin.com/linux/latest/source/mm/pagewalk.c#L103

If a huge pud is concurrently faulted in here, it will immediatly get split without getting processed in pud_entry(). An external lock would protect against that, but that's perhaps a bug in the pagewalk code? For pmds the situation is not the same since when pte_entry is used, all pmds will unconditionally get split.

There's a similar more scary race in

https://elixir.bootlin.com/linux/latest/source/mm/memory.c#L3931

It looks like if a concurrent thread faults in a huge pud just after the test for pud_none in that pmd_alloc, things might go pretty bad.

/Thomas