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SubjectRe: [v4 PATCH 2/2] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind
On Mon, 22 Jul 2019 09:25:09 +0200 Vlastimil Babka <vbabka@suse.cz> wrote:

> > since there may be pages off LRU temporarily. We should migrate other
> > pages if MPOL_MF_MOVE* is specified. Set has_unmovable flag if some
> > paged could not be not moved, then return -EIO for mbind() eventually.
> >
> > With this change the above test would return -EIO as expected.
> >
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Mel Gorman <mgorman@techsingularity.net>
> > Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Thanks.

I'm a bit surprised that this doesn't have a cc:stable. Did we
consider that?

Also, is this patch dependent upon "mm: mempolicy: make the behavior
consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified"?
Doesn't look that way..

Also, I have a note that you had concerns with "mm: mempolicy: make the
behavior consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were
specified". What is the status now?


From: Yang Shi <yang.shi@linux.alibaba.com>
Subject: mm: mempolicy: make the behavior consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified

When both MPOL_MF_MOVE* and MPOL_MF_STRICT was specified, mbind() should
try best to migrate misplaced pages, if some of the pages could not be
migrated, then return -EIO.

There are three different sub-cases:
1. vma is not migratable
2. vma is migratable, but there are unmovable pages
3. vma is migratable, pages are movable, but migrate_pages() fails

If #1 happens, kernel would just abort immediately, then return -EIO,
after the commit a7f40cfe3b7ada57af9b62fd28430eeb4a7cfcb7 ("mm: mempolicy:
make mbind() return -EIO when MPOL_MF_STRICT is specified").

If #3 happens, kernel would set policy and migrate pages with best-effort,
but won't rollback the migrated pages and reset the policy back.

Before that commit, they behaves in the same way. It'd better to keep
their behavior consistent. But, rolling back the migrated pages and
resetting the policy back sounds not feasible, so just make #1 behave as
same as #3.

Userspace will know that not everything was successfully migrated (via
-EIO), and can take whatever steps it deems necessary - attempt rollback,
determine which exact page(s) are violating the policy, etc.

Make queue_pages_range() return 1 to indicate there are unmovable pages or
vma is not migratable.

The #2 is not handled correctly in the current kernel, the following patch
will fix it.

Link: http://lkml.kernel.org/r/1561162809-59140-2-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/mempolicy.c | 84 +++++++++++++++++++++++++++++++++--------------
1 file changed, 60 insertions(+), 24 deletions(-)

--- a/mm/mempolicy.c~mm-mempolicy-make-the-behavior-consistent-when-mpol_mf_move-and-mpol_mf_strict-were-specified
+++ a/mm/mempolicy.c
@@ -429,11 +429,14 @@ static inline bool queue_pages_required(
}

/*
- * queue_pages_pmd() has three possible return values:
+ * queue_pages_pmd() has four possible return values:
+ * 2 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
+ * specified.
* 1 - pages are placed on the right node or queued successfully.
* 0 - THP was split.
- * -EIO - is migration entry or MPOL_MF_STRICT was specified and an existing
- * page was already on a node that does not follow the policy.
+ * -EIO - is migration entry or only MPOL_MF_STRICT was specified and an
+ * existing page was already on a node that does not follow the
+ * policy.
*/
static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
unsigned long end, struct mm_walk *walk)
@@ -463,7 +466,7 @@ static int queue_pages_pmd(pmd_t *pmd, s
/* go to thp migration */
if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
if (!vma_migratable(walk->vma)) {
- ret = -EIO;
+ ret = 2;
goto unlock;
}

@@ -488,16 +491,29 @@ static int queue_pages_pte_range(pmd_t *
struct queue_pages *qp = walk->private;
unsigned long flags = qp->flags;
int ret;
+ bool has_unmovable = false;
pte_t *pte;
spinlock_t *ptl;

ptl = pmd_trans_huge_lock(pmd, vma);
if (ptl) {
ret = queue_pages_pmd(pmd, ptl, addr, end, walk);
- if (ret > 0)
+ switch (ret) {
+ /* THP was split, fall through to pte walk */
+ case 0:
+ break;
+ /* Pages are placed on the right node or queued successfully */
+ case 1:
return 0;
- else if (ret < 0)
+ /*
+ * Met unmovable pages, MPOL_MF_MOVE* & MPOL_MF_STRICT
+ * were specified.
+ */
+ case 2:
+ return 1;
+ case -EIO:
return ret;
+ }
}

if (pmd_trans_unstable(pmd))
@@ -519,14 +535,21 @@ static int queue_pages_pte_range(pmd_t *
if (!queue_pages_required(page, qp))
continue;
if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
- if (!vma_migratable(vma))
+ /* MPOL_MF_STRICT must be specified if we get here */
+ if (!vma_migratable(vma)) {
+ has_unmovable |= true;
break;
+ }
migrate_page_add(page, qp->pagelist, flags);
} else
break;
}
pte_unmap_unlock(pte - 1, ptl);
cond_resched();
+
+ if (has_unmovable)
+ return 1;
+
return addr != end ? -EIO : 0;
}

@@ -639,7 +662,13 @@ static int queue_pages_test_walk(unsigne
*
* If pages found in a given range are on a set of nodes (determined by
* @nodes and @flags,) it's isolated and queued to the pagelist which is
- * passed via @private.)
+ * passed via @private.
+ *
+ * queue_pages_range() has three possible return values:
+ * 1 - there is unmovable page, but MPOL_MF_MOVE* & MPOL_MF_STRICT were
+ * specified.
+ * 0 - queue pages successfully or no misplaced page.
+ * -EIO - there is misplaced page and only MPOL_MF_STRICT was specified.
*/
static int
queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
@@ -1182,6 +1211,7 @@ static long do_mbind(unsigned long start
struct mempolicy *new;
unsigned long end;
int err;
+ int ret;
LIST_HEAD(pagelist);

if (flags & ~(unsigned long)MPOL_MF_VALID)
@@ -1243,26 +1273,32 @@ static long do_mbind(unsigned long start
if (err)
goto mpol_out;

- err = queue_pages_range(mm, start, end, nmask,
+ ret = queue_pages_range(mm, start, end, nmask,
flags | MPOL_MF_INVERT, &pagelist);
- if (!err)
- err = mbind_range(mm, start, end, new);

- if (!err) {
- int nr_failed = 0;
+ if (ret < 0)
+ err = -EIO;
+ else {
+ err = mbind_range(mm, start, end, new);

- if (!list_empty(&pagelist)) {
- WARN_ON_ONCE(flags & MPOL_MF_LAZY);
- nr_failed = migrate_pages(&pagelist, new_page, NULL,
- start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND);
- if (nr_failed)
- putback_movable_pages(&pagelist);
- }
+ if (!err) {
+ int nr_failed = 0;

- if (nr_failed && (flags & MPOL_MF_STRICT))
- err = -EIO;
- } else
- putback_movable_pages(&pagelist);
+ if (!list_empty(&pagelist)) {
+ WARN_ON_ONCE(flags & MPOL_MF_LAZY);
+ nr_failed = migrate_pages(&pagelist, new_page,
+ NULL, start, MIGRATE_SYNC,
+ MR_MEMPOLICY_MBIND);
+ if (nr_failed)
+ putback_movable_pages(&pagelist);
+ }
+
+ if ((ret > 0) ||
+ (nr_failed && (flags & MPOL_MF_STRICT)))
+ err = -EIO;
+ } else
+ putback_movable_pages(&pagelist);
+ }

up_write(&mm->mmap_sem);
mpol_out:
_
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