Re: [v2 PATCH 1/2] mm: mempolicy: make the behavior consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified
From: Yang Shi
Date: Tue Jul 16 2019 - 13:18:37 EST
On 7/16/19 1:12 AM, Vlastimil Babka wrote:
On 6/22/19 2:20 AM, Yang Shi wrote:
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.
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Agreed with the goal, but I think there's a bug, and room for improvement.
---
mm/mempolicy.c | 86 +++++++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 61 insertions(+), 25 deletions(-)
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 01600d8..b50039c 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -429,11 +429,14 @@ static inline bool queue_pages_required(struct page *page,
}
/*
- * 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.
I think if you renumbered these, it would be more consistent with
queue_pages_pte_range() and simplify the code there.
0 - pages on right node/queued
1 - unmovable page with right flags specified
2 - THP 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, spinlock_t *ptl, unsigned long addr,
/* 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 *pmd, unsigned long addr,
Perhaps this function now also deserves a list of possible return values.
Sure, will add some comments to elaborate the return values.
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) {
With renumbering suggested above, this could be:
if (ret != 2)
return 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 *pmd, unsigned long addr,
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(unsigned long start, unsigned long end,
*
* 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, unsigned long len,
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, unsigned long len,
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 (!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)
+ err = -EIO;
I think after your patch, you miss putback_movable_pages() in cases
where some were queued, and later the walk returned -EIO. The previous
code doesn't miss it, but it's also not obvious due to the multiple if
(!err) checks. I would rewrite it some thing like this:
if (ret < 0) {
putback_movable_pages(&pagelist);
err = ret;
goto mmap_out; // a new label above up_write()
}
Yes, the old code had putback_movable_pages called when !err. But, I
think that is for error handling of mbind_range() if I understand it
correctly since if queue_pages_range() returns -EIO (only MPOL_MF_STRICT
was specified and there was misplaced page) that page list should be
empty . The old code should checked whether that list is empty or not.
So, in the new code I just removed that.
The rest can have reduced identation now.
Yes, the goto does eliminate the extra indentation.
+ else {
+ err = mbind_range(mm, start, end, new);
- if (nr_failed && (flags & MPOL_MF_STRICT))
- err = -EIO;
- } else
- putback_movable_pages(&pagelist);
+ if (!err) {
+ int nr_failed = 0;
+
+ 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);
While at it, IIRC the kernel style says that when the 'if' part uses
'{ }' then the 'else' part should as well, and it shouldn't be mixed.
Really? The old code doesn't have '{ }' for else, and checkpatch doesn't
report any error or warning.
Thanks,
Yang
Thanks,
Vlastimil
+ }
up_write(&mm->mmap_sem);
mpol_out: