Commit d952b791 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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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: default avatarHugh Dickins <>
Acked-by: default avatarIzik Eidus <>
Cc: Andrea Arcangeli <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 81464e30
......@@ -294,10 +294,10 @@ static inline int in_stable_tree(struct rmap_item *rmap_item)
* 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 {
......@@ -310,9 +310,36 @@ static void break_ksm(struct vm_area_struct *vma, unsigned long addr)
} while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS)));
/* Which leaves us looping there if VM_FAULT_OOM: hmmm... */
* 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 @@ static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
* 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 = 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;
mm_slot = list_entry(,
struct mm_slot, mm_list);
list_for_each_entry(mm_slot, &ksm_mm_head.mm_list, mm_list) {
while (mm_slot != &ksm_mm_head) {
mm = mm_slot->mm;
for (vma = mm->mmap; vma; vma = vma->vm_next) {
if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
err = unmerge_ksm_pages(vma,
vma->vm_start, vma->vm_end);
if (err) {
goto out;
remove_trailing_rmap_items(mm_slot, mm_slot->;
mm_slot = list_entry(mm_slot->,
struct mm_slot, mm_list);
ksm_scan.seqnr = 0;
if (ksm_scan.mm_slot != &ksm_mm_head) {
ksm_scan.mm_slot = &ksm_mm_head;
ksm_scan.mm_slot = &ksm_mm_head;
return err;
static void remove_mm_from_lists(struct mm_struct *mm)
......@@ -1051,6 +1100,8 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
* 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 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
unsigned long end, int advice, unsigned long *vm_flags)
struct mm_struct *mm = vma->vm_mm;
int err;
switch (advice) {
......@@ -1298,9 +1350,11 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
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;
......@@ -1309,8 +1363,11 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
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;
......@@ -1441,8 +1498,13 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
if (ksm_run != flags) {
ksm_run = flags;
if (flags & KSM_RUN_UNMERGE)
if (flags & KSM_RUN_UNMERGE) {
err = unmerge_and_remove_all_rmap_items();
if (err) {
ksm_run = KSM_RUN_STOP;
count = err;
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