ksm: fix endless loop on oom
break_ksm has been looping endlessly ignoring VM_FAULT_OOM: that should
only be a problem for ksmd when a memory control group imposes limits
(normally the OOM killer will kill others with an mm until it succeeds);
but in general (especially for MADV_UNMERGEABLE and KSM_RUN_UNMERGE) we
do need to route the error (or kill) back to the caller (or sighandling).
Test signal_pending in unmerge_ksm_pages, which could be a lengthy
procedure if it has to spill into swap: returning -ERESTARTSYS so that
trivial signals will restart but fatals will terminate (is that right?
we do different things in different places in mm, none exactly this).
unmerge_and_remove_all_rmap_items was forgetting to lock when going
down the mm_list: fix that. Whether it's successful or not, reset
ksm_scan cursor to head; but only if it's successful, reset seqnr
(shown in full_scans) - page counts will have gone down to zero.
This patch leaves a significant OOM deadlock, but it's a good step
on the way, and that deadlock is fixed in a subsequent patch.
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/ksm.c b/mm/ksm.c
index c49bb71..d9e3cfc 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -294,10 +294,10 @@
* Could a ksm page appear anywhere else? Actually yes, in a VM_PFNMAP
* mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
*/
-static void break_ksm(struct vm_area_struct *vma, unsigned long addr)
+static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
struct page *page;
- int ret;
+ int ret = 0;
do {
cond_resched();
@@ -310,9 +310,36 @@
else
ret = VM_FAULT_WRITE;
put_page(page);
- } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS)));
-
- /* Which leaves us looping there if VM_FAULT_OOM: hmmm... */
+ } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_OOM)));
+ /*
+ * We must loop because handle_mm_fault() may back out if there's
+ * any difficulty e.g. if pte accessed bit gets updated concurrently.
+ *
+ * VM_FAULT_WRITE is what we have been hoping for: it indicates that
+ * COW has been broken, even if the vma does not permit VM_WRITE;
+ * but note that a concurrent fault might break PageKsm for us.
+ *
+ * VM_FAULT_SIGBUS could occur if we race with truncation of the
+ * backing file, which also invalidates anonymous pages: that's
+ * okay, that truncation will have unmapped the PageKsm for us.
+ *
+ * VM_FAULT_OOM: at the time of writing (late July 2009), setting
+ * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
+ * current task has TIF_MEMDIE set, and will be OOM killed on return
+ * to user; and ksmd, having no mm, would never be chosen for that.
+ *
+ * But if the mm is in a limited mem_cgroup, then the fault may fail
+ * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
+ * even ksmd can fail in this way - though it's usually breaking ksm
+ * just to undo a merge it made a moment before, so unlikely to oom.
+ *
+ * That's a pity: we might therefore have more kernel pages allocated
+ * than we're counting as nodes in the stable tree; but ksm_do_scan
+ * will retry to break_cow on each pass, so should recover the page
+ * in due course. The important thing is to not let VM_MERGEABLE
+ * be cleared while any such pages might remain in the area.
+ */
+ return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
}
static void break_cow(struct mm_struct *mm, unsigned long addr)
@@ -462,39 +489,61 @@
* to the next pass of ksmd - consider, for example, how ksmd might be
* in cmp_and_merge_page on one of the rmap_items we would be removing.
*/
-static void unmerge_ksm_pages(struct vm_area_struct *vma,
- unsigned long start, unsigned long end)
+static int unmerge_ksm_pages(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end)
{
unsigned long addr;
+ int err = 0;
- for (addr = start; addr < end; addr += PAGE_SIZE)
- break_ksm(vma, addr);
+ for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
+ if (signal_pending(current))
+ err = -ERESTARTSYS;
+ else
+ err = break_ksm(vma, addr);
+ }
+ return err;
}
-static void unmerge_and_remove_all_rmap_items(void)
+static int unmerge_and_remove_all_rmap_items(void)
{
struct mm_slot *mm_slot;
struct mm_struct *mm;
struct vm_area_struct *vma;
+ int err = 0;
- list_for_each_entry(mm_slot, &ksm_mm_head.mm_list, mm_list) {
+ spin_lock(&ksm_mmlist_lock);
+ mm_slot = list_entry(ksm_mm_head.mm_list.next,
+ struct mm_slot, mm_list);
+ spin_unlock(&ksm_mmlist_lock);
+
+ while (mm_slot != &ksm_mm_head) {
mm = mm_slot->mm;
down_read(&mm->mmap_sem);
for (vma = mm->mmap; vma; vma = vma->vm_next) {
if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
continue;
- unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
+ err = unmerge_ksm_pages(vma,
+ vma->vm_start, vma->vm_end);
+ if (err) {
+ up_read(&mm->mmap_sem);
+ goto out;
+ }
}
remove_trailing_rmap_items(mm_slot, mm_slot->rmap_list.next);
up_read(&mm->mmap_sem);
+
+ spin_lock(&ksm_mmlist_lock);
+ mm_slot = list_entry(mm_slot->mm_list.next,
+ struct mm_slot, mm_list);
+ spin_unlock(&ksm_mmlist_lock);
}
+ ksm_scan.seqnr = 0;
+out:
spin_lock(&ksm_mmlist_lock);
- if (ksm_scan.mm_slot != &ksm_mm_head) {
- ksm_scan.mm_slot = &ksm_mm_head;
- ksm_scan.seqnr++;
- }
+ ksm_scan.mm_slot = &ksm_mm_head;
spin_unlock(&ksm_mmlist_lock);
+ return err;
}
static void remove_mm_from_lists(struct mm_struct *mm)
@@ -1051,6 +1100,8 @@
/*
* A ksm page might have got here by fork, but its other
* references have already been removed from the stable tree.
+ * Or it might be left over from a break_ksm which failed
+ * when the mem_cgroup had reached its limit: try again now.
*/
if (PageKsm(page))
break_cow(rmap_item->mm, rmap_item->address);
@@ -1286,6 +1337,7 @@
unsigned long end, int advice, unsigned long *vm_flags)
{
struct mm_struct *mm = vma->vm_mm;
+ int err;
switch (advice) {
case MADV_MERGEABLE:
@@ -1298,9 +1350,11 @@
VM_MIXEDMAP | VM_SAO))
return 0; /* just ignore the advice */
- if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
- if (__ksm_enter(mm) < 0)
- return -EAGAIN;
+ if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
+ err = __ksm_enter(mm);
+ if (err)
+ return err;
+ }
*vm_flags |= VM_MERGEABLE;
break;
@@ -1309,8 +1363,11 @@
if (!(*vm_flags & VM_MERGEABLE))
return 0; /* just ignore the advice */
- if (vma->anon_vma)
- unmerge_ksm_pages(vma, start, end);
+ if (vma->anon_vma) {
+ err = unmerge_ksm_pages(vma, start, end);
+ if (err)
+ return err;
+ }
*vm_flags &= ~VM_MERGEABLE;
break;
@@ -1441,8 +1498,13 @@
mutex_lock(&ksm_thread_mutex);
if (ksm_run != flags) {
ksm_run = flags;
- if (flags & KSM_RUN_UNMERGE)
- unmerge_and_remove_all_rmap_items();
+ if (flags & KSM_RUN_UNMERGE) {
+ err = unmerge_and_remove_all_rmap_items();
+ if (err) {
+ ksm_run = KSM_RUN_STOP;
+ count = err;
+ }
+ }
}
mutex_unlock(&ksm_thread_mutex);