From: xu xin <[email protected]>
use_zero_pages may be very useful, not just because of cache colouring
as described in doc, but also because use_zero_pages can accelerate
merging empty pages when there are plenty of empty pages (full of zeros)
as the time of page-by-page comparisons (unstable_tree_search_insert) is
saved.
But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and
other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing
will *not* unshare the shared zeropage as placed by KSM (which may be
against the MADV_UNMERGEABLE documentation at least).
To not blindly unshare all shared zero_pages in applicable VMAs, the patch
introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those
shared zero_pages. and guarantee that these rmap_items will be not freed
during the time of zero_pages not being writing, so we can only unshare
the *KSM-placed* zero_pages.
The patch will not degrade the performance of use_zero_pages as it doesn't
change the way of merging empty pages in use_zero_pages's feature.
Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
Reported-by: David Hildenbrand <[email protected]>
Cc: Claudio Imbrenda <[email protected]>
Cc: Xuexin Jiang <[email protected]>
Signed-off-by: xu xin <[email protected]>
Co-developed-by: Xiaokai Ran <[email protected]>
Signed-off-by: Xiaokai Ran <[email protected]>
Co-developed-by: Yang Yang <[email protected]>
Signed-off-by: Yang Yang <[email protected]>
Signed-off-by: xu xin <[email protected]>
---
mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++-------------
1 file changed, 105 insertions(+), 31 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 13c60f1071d8..e351d7b6d15e 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -213,6 +213,7 @@ struct ksm_rmap_item {
#define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
#define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
#define STABLE_FLAG 0x200 /* is listed from the stable tree */
+#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */
/* The stable and unstable tree heads */
static struct rb_root one_stable_tree[1] = { RB_ROOT };
@@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void)
return rmap_item;
}
-static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
-{
- ksm_rmap_items--;
- rmap_item->mm->ksm_rmap_items--;
- rmap_item->mm = NULL; /* debug safety */
- kmem_cache_free(rmap_item_cache, rmap_item);
-}
-
static inline struct ksm_stable_node *alloc_stable_node(void)
{
/*
@@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
}
/*
- * We use break_ksm to break COW on a ksm page: it's a stripped down
+ * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
+ * happen when enabling use_zero_pages): it's a stripped down
*
* if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
* put_page(page);
@@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
* of the process that owns 'vma'. We also do not want to enforce
* protection keys here anyway.
*/
-static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
+static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
+ bool ksm_check_bypass)
{
struct page *page;
vm_fault_t ret = 0;
@@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
ret = handle_mm_fault(vma, addr,
FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
NULL);
+ else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
+ /*
+ * Although it's not ksm page, it's zero page as placed by
+ * KSM use_zero_page, so we should unshare it when
+ * ksm_check_bypass is true.
+ */
+ ret = handle_mm_fault(vma, addr,
+ FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
+ NULL);
+ }
else
ret = VM_FAULT_WRITE;
put_page(page);
@@ -496,6 +501,11 @@ static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
return vma;
}
+/*
+ * Note: Don't call break_cow() in the context protected by
+ * mmap_read_lock(), which may cause dead lock because inside
+ * break_cow mmap_read_lock exists.
+ */
static void break_cow(struct ksm_rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
@@ -511,10 +521,35 @@ static void break_cow(struct ksm_rmap_item *rmap_item)
mmap_read_lock(mm);
vma = find_mergeable_vma(mm, addr);
if (vma)
- break_ksm(vma, addr);
+ break_ksm(vma, addr, false);
mmap_read_unlock(mm);
}
+/* Only called when rmap_item->address is with ZERO_PAGE_FLAG */
+static inline int unshare_zero_pages(struct ksm_rmap_item *rmap_item)
+{
+ struct mm_struct *mm = rmap_item->mm;
+ struct vm_area_struct *vma;
+ unsigned long addr = rmap_item->address;
+ int err = -EFAULT;
+
+ vma = vma_lookup(mm, addr);
+ if (vma)
+ err = break_ksm(vma, addr, true);
+
+ return err;
+}
+
+static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
+{
+ if (rmap_item->address & ZERO_PAGE_FLAG)
+ unshare_zero_pages(rmap_item);
+ ksm_rmap_items--;
+ rmap_item->mm->ksm_rmap_items--;
+ rmap_item->mm = NULL; /* debug safety */
+ kmem_cache_free(rmap_item_cache, rmap_item);
+}
+
static struct page *get_mergeable_page(struct ksm_rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
@@ -825,7 +860,7 @@ static int unmerge_ksm_pages(struct vm_area_struct *vma,
if (signal_pending(current))
err = -ERESTARTSYS;
else
- err = break_ksm(vma, addr);
+ err = break_ksm(vma, addr, false);
}
return err;
}
@@ -2017,6 +2052,36 @@ static void stable_tree_append(struct ksm_rmap_item *rmap_item,
rmap_item->mm->ksm_merging_pages++;
}
+static int try_to_merge_with_kernel_zero_page(struct mm_struct *mm,
+ struct ksm_rmap_item *rmap_item,
+ struct page *page)
+{
+ int err = 0;
+
+ if (!(rmap_item->address & ZERO_PAGE_FLAG)) {
+ struct vm_area_struct *vma;
+
+ mmap_read_lock(mm);
+ vma = find_mergeable_vma(mm, rmap_item->address);
+ if (vma) {
+ err = try_to_merge_one_page(vma, page,
+ ZERO_PAGE(rmap_item->address));
+ } else {
+ /* If the vma is out of date, we do not need to continue. */
+ err = 0;
+ }
+ mmap_read_unlock(mm);
+ /*
+ * In case of failure, the page was not really empty, so we
+ * need to continue. Otherwise we're done.
+ */
+ if (!err)
+ rmap_item->address |= ZERO_PAGE_FLAG;
+ }
+
+ return err;
+}
+
/*
* cmp_and_merge_page - first see if page can be merged into the stable tree;
* if not, compare checksum to previous and if it's the same, see if page can
@@ -2101,29 +2166,21 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
* Same checksum as an empty page. We attempt to merge it with the
* appropriate zero page if the user enabled this via sysfs.
*/
- if (ksm_use_zero_pages && (checksum == zero_checksum)) {
- struct vm_area_struct *vma;
-
- mmap_read_lock(mm);
- vma = find_mergeable_vma(mm, rmap_item->address);
- if (vma) {
- err = try_to_merge_one_page(vma, page,
- ZERO_PAGE(rmap_item->address));
- } else {
+ if (ksm_use_zero_pages) {
+ if (checksum == zero_checksum) {
+ /* If success, just return. Otherwise, continue */
+ if (!try_to_merge_with_kernel_zero_page(mm, rmap_item, page))
+ return;
+ } else if (rmap_item->address & ZERO_PAGE_FLAG) {
/*
- * If the vma is out of date, we do not need to
- * continue.
+ * The page now is not kernel zero page (COW happens to it)
+ * but the flag of its rmap_item is still zero-page, so need
+ * to reset the flag and update the corresponding count.
*/
- err = 0;
+ rmap_item->address &= PAGE_MASK;
}
- mmap_read_unlock(mm);
- /*
- * In case of failure, the page was not really empty, so we
- * need to continue. Otherwise we're done.
- */
- if (!err)
- return;
}
+
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {
@@ -2337,6 +2394,23 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
mmap_read_unlock(mm);
return rmap_item;
}
+ /*
+ * Because we want to monitor ksm zero pages which is
+ * non-anonymous, we must try to return the rmap_items
+ * of those kernel zero pages which replaces its
+ * original anonymous empty page due to use_zero_pages's
+ * feature.
+ */
+ if (is_zero_pfn(page_to_pfn(*page))) {
+ rmap_item = try_to_get_old_rmap_item(ksm_scan.address,
+ ksm_scan.rmap_list);
+ if (rmap_item && (rmap_item->address & ZERO_PAGE_FLAG)) {
+ ksm_scan.rmap_list = &rmap_item->rmap_list;
+ ksm_scan.address += PAGE_SIZE;
+ mmap_read_unlock(mm);
+ return rmap_item;
+ }
+ }
next_page:
put_page(*page);
ksm_scan.address += PAGE_SIZE;
--
2.25.1
On 11.10.22 04:22, [email protected] wrote:
> From: xu xin <[email protected]>
>
> use_zero_pages may be very useful, not just because of cache colouring
> as described in doc, but also because use_zero_pages can accelerate
> merging empty pages when there are plenty of empty pages (full of zeros)
> as the time of page-by-page comparisons (unstable_tree_search_insert) is
> saved.
>
> But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and
> other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing
> will *not* unshare the shared zeropage as placed by KSM (which may be
> against the MADV_UNMERGEABLE documentation at least).
>
> To not blindly unshare all shared zero_pages in applicable VMAs, the patch
> introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those
> shared zero_pages. and guarantee that these rmap_items will be not freed
> during the time of zero_pages not being writing, so we can only unshare
> the *KSM-placed* zero_pages.
>
> The patch will not degrade the performance of use_zero_pages as it doesn't
> change the way of merging empty pages in use_zero_pages's feature.
>
> Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
> Reported-by: David Hildenbrand <[email protected]>
> Cc: Claudio Imbrenda <[email protected]>
> Cc: Xuexin Jiang <[email protected]>
> Signed-off-by: xu xin <[email protected]>
> Co-developed-by: Xiaokai Ran <[email protected]>
> Signed-off-by: Xiaokai Ran <[email protected]>
> Co-developed-by: Yang Yang <[email protected]>
> Signed-off-by: Yang Yang <[email protected]>
> Signed-off-by: xu xin <[email protected]>
> ---
> mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 105 insertions(+), 31 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 13c60f1071d8..e351d7b6d15e 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -213,6 +213,7 @@ struct ksm_rmap_item {
> #define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
> #define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
> #define STABLE_FLAG 0x200 /* is listed from the stable tree */
> +#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */
>
> /* The stable and unstable tree heads */
> static struct rb_root one_stable_tree[1] = { RB_ROOT };
> @@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void)
> return rmap_item;
> }
>
> -static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
> -{
> - ksm_rmap_items--;
> - rmap_item->mm->ksm_rmap_items--;
> - rmap_item->mm = NULL; /* debug safety */
> - kmem_cache_free(rmap_item_cache, rmap_item);
> -}
> -
> static inline struct ksm_stable_node *alloc_stable_node(void)
> {
> /*
> @@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
> }
>
> /*
> - * We use break_ksm to break COW on a ksm page: it's a stripped down
> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
> + * happen when enabling use_zero_pages): it's a stripped down
> *
> * if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
> * put_page(page);
> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
> * of the process that owns 'vma'. We also do not want to enforce
> * protection keys here anyway.
> */
> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
> + bool ksm_check_bypass)
> {
> struct page *page;
> vm_fault_t ret = 0;
> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
> ret = handle_mm_fault(vma, addr,
> FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
> NULL);
> + else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
> + /*
> + * Although it's not ksm page, it's zero page as placed by
> + * KSM use_zero_page, so we should unshare it when
> + * ksm_check_bypass is true.
> + */
> + ret = handle_mm_fault(vma, addr,
> + FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
> + NULL);
> + }
Please don't duplicate that page fault triggering code.
Also, please be aware that this collides with
https://lkml.kernel.org/r/[email protected]
Adjustments should be comparatively easy.
--
Thanks,
David / dhildenb
On 21.10.22 12:17, David Hildenbrand wrote:
> On 11.10.22 04:22, [email protected] wrote:
>> From: xu xin <[email protected]>
>>
>> use_zero_pages may be very useful, not just because of cache colouring
>> as described in doc, but also because use_zero_pages can accelerate
>> merging empty pages when there are plenty of empty pages (full of zeros)
>> as the time of page-by-page comparisons (unstable_tree_search_insert) is
>> saved.
>>
>> But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and
>> other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing
>> will *not* unshare the shared zeropage as placed by KSM (which may be
>> against the MADV_UNMERGEABLE documentation at least).
>>
>> To not blindly unshare all shared zero_pages in applicable VMAs, the patch
>> introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those
>> shared zero_pages. and guarantee that these rmap_items will be not freed
>> during the time of zero_pages not being writing, so we can only unshare
>> the *KSM-placed* zero_pages.
>>
>> The patch will not degrade the performance of use_zero_pages as it doesn't
>> change the way of merging empty pages in use_zero_pages's feature.
>>
>> Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
>> Reported-by: David Hildenbrand <[email protected]>
>> Cc: Claudio Imbrenda <[email protected]>
>> Cc: Xuexin Jiang <[email protected]>
>> Signed-off-by: xu xin <[email protected]>
>> Co-developed-by: Xiaokai Ran <[email protected]>
>> Signed-off-by: Xiaokai Ran <[email protected]>
>> Co-developed-by: Yang Yang <[email protected]>
>> Signed-off-by: Yang Yang <[email protected]>
>> Signed-off-by: xu xin <[email protected]>
>> ---
>> mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++-------------
>> 1 file changed, 105 insertions(+), 31 deletions(-)
>>
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 13c60f1071d8..e351d7b6d15e 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -213,6 +213,7 @@ struct ksm_rmap_item {
>> #define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
>> #define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
>> #define STABLE_FLAG 0x200 /* is listed from the stable tree */
>> +#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */
>>
>> /* The stable and unstable tree heads */
>> static struct rb_root one_stable_tree[1] = { RB_ROOT };
>> @@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void)
>> return rmap_item;
>> }
>>
>> -static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
>> -{
>> - ksm_rmap_items--;
>> - rmap_item->mm->ksm_rmap_items--;
>> - rmap_item->mm = NULL; /* debug safety */
>> - kmem_cache_free(rmap_item_cache, rmap_item);
>> -}
>> -
>> static inline struct ksm_stable_node *alloc_stable_node(void)
>> {
>> /*
>> @@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>> }
>>
>> /*
>> - * We use break_ksm to break COW on a ksm page: it's a stripped down
>> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
>> + * happen when enabling use_zero_pages): it's a stripped down
>> *
>> * if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
>> * put_page(page);
>> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>> * of the process that owns 'vma'. We also do not want to enforce
>> * protection keys here anyway.
>> */
>> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
>> + bool ksm_check_bypass)
>> {
>> struct page *page;
>> vm_fault_t ret = 0;
>> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>> ret = handle_mm_fault(vma, addr,
>> FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>> NULL);
>> + else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
>> + /*
>> + * Although it's not ksm page, it's zero page as placed by
>> + * KSM use_zero_page, so we should unshare it when
>> + * ksm_check_bypass is true.
>> + */
>> + ret = handle_mm_fault(vma, addr,
>> + FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>> + NULL);
>> + }
>
> Please don't duplicate that page fault triggering code.
>
> Also, please be aware that this collides with
>
> https://lkml.kernel.org/r/[email protected]
>
> Adjustments should be comparatively easy.
... except that I'm still working on FAULT_FLAG_UNSHARE support for the
shared zeropage. That will be posted soonish (within next 2 weeks).
--
Thanks,
David / dhildenb
On 21.10.22 14:54, David Hildenbrand wrote:
> On 21.10.22 12:17, David Hildenbrand wrote:
>> On 11.10.22 04:22, [email protected] wrote:
>>> From: xu xin <[email protected]>
>>>
>>> use_zero_pages may be very useful, not just because of cache colouring
>>> as described in doc, but also because use_zero_pages can accelerate
>>> merging empty pages when there are plenty of empty pages (full of zeros)
>>> as the time of page-by-page comparisons (unstable_tree_search_insert) is
>>> saved.
>>>
>>> But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and
>>> other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing
>>> will *not* unshare the shared zeropage as placed by KSM (which may be
>>> against the MADV_UNMERGEABLE documentation at least).
>>>
>>> To not blindly unshare all shared zero_pages in applicable VMAs, the patch
>>> introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those
>>> shared zero_pages. and guarantee that these rmap_items will be not freed
>>> during the time of zero_pages not being writing, so we can only unshare
>>> the *KSM-placed* zero_pages.
>>>
>>> The patch will not degrade the performance of use_zero_pages as it doesn't
>>> change the way of merging empty pages in use_zero_pages's feature.
>>>
>>> Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
>>> Reported-by: David Hildenbrand <[email protected]>
>>> Cc: Claudio Imbrenda <[email protected]>
>>> Cc: Xuexin Jiang <[email protected]>
>>> Signed-off-by: xu xin <[email protected]>
>>> Co-developed-by: Xiaokai Ran <[email protected]>
>>> Signed-off-by: Xiaokai Ran <[email protected]>
>>> Co-developed-by: Yang Yang <[email protected]>
>>> Signed-off-by: Yang Yang <[email protected]>
>>> Signed-off-by: xu xin <[email protected]>
>>> ---
>>> mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++-------------
>>> 1 file changed, 105 insertions(+), 31 deletions(-)
>>>
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index 13c60f1071d8..e351d7b6d15e 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -213,6 +213,7 @@ struct ksm_rmap_item {
>>> #define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
>>> #define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
>>> #define STABLE_FLAG 0x200 /* is listed from the stable tree */
>>> +#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */
>>>
>>> /* The stable and unstable tree heads */
>>> static struct rb_root one_stable_tree[1] = { RB_ROOT };
>>> @@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void)
>>> return rmap_item;
>>> }
>>>
>>> -static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
>>> -{
>>> - ksm_rmap_items--;
>>> - rmap_item->mm->ksm_rmap_items--;
>>> - rmap_item->mm = NULL; /* debug safety */
>>> - kmem_cache_free(rmap_item_cache, rmap_item);
>>> -}
>>> -
>>> static inline struct ksm_stable_node *alloc_stable_node(void)
>>> {
>>> /*
>>> @@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>>> }
>>>
>>> /*
>>> - * We use break_ksm to break COW on a ksm page: it's a stripped down
>>> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
>>> + * happen when enabling use_zero_pages): it's a stripped down
>>> *
>>> * if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
>>> * put_page(page);
>>> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>>> * of the process that owns 'vma'. We also do not want to enforce
>>> * protection keys here anyway.
>>> */
>>> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
>>> + bool ksm_check_bypass)
>>> {
>>> struct page *page;
>>> vm_fault_t ret = 0;
>>> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>> ret = handle_mm_fault(vma, addr,
>>> FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>> NULL);
>>> + else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
>>> + /*
>>> + * Although it's not ksm page, it's zero page as placed by
>>> + * KSM use_zero_page, so we should unshare it when
>>> + * ksm_check_bypass is true.
>>> + */
>>> + ret = handle_mm_fault(vma, addr,
>>> + FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>> + NULL);
>>> + }
>>
>> Please don't duplicate that page fault triggering code.
>>
>> Also, please be aware that this collides with
>>
>> https://lkml.kernel.org/r/[email protected]
>>
>> Adjustments should be comparatively easy.
>
> ... except that I'm still working on FAULT_FLAG_UNSHARE support for the
> shared zeropage. That will be posted soonish (within next 2 weeks).
>
Posted: https://lkml.kernel.org/r/[email protected]
With that, we can use FAULT_FLAG_UNSHARE also to break COW on the shared
zeropage.
--
Thanks,
David / dhildenb
>>>> - * We use break_ksm to break COW on a ksm page: it's a stripped down
>>>> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
>>>> + * happen when enabling use_zero_pages): it's a stripped down
>>>> *
>>>> * if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
>>>> * put_page(page);
>>>> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>>>> * of the process that owns 'vma'. We also do not want to enforce
>>>> * protection keys here anyway.
>>>> */
>>>> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>>> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
>>>> + bool ksm_check_bypass)
>>>> {
>>>> struct page *page;
>>>> vm_fault_t ret = 0;
>>>> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>>> ret = handle_mm_fault(vma, addr,
>>>> FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>>> NULL);
>>>> + else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
>>>> + /*
>>>> + * Although it's not ksm page, it's zero page as placed by
>>>> + * KSM use_zero_page, so we should unshare it when
>>>> + * ksm_check_bypass is true.
>>>> + */
>>>> + ret = handle_mm_fault(vma, addr,
>>>> + FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>>> + NULL);
>>>> + }
>>>
>>> Please don't duplicate that page fault triggering code.
>>>
>>> Also, please be aware that this collides with
>>>
>>> https://lkml.kernel.org/r/[email protected]
>>>
>>> Adjustments should be comparatively easy.
>>
>> ... except that I'm still working on FAULT_FLAG_UNSHARE support for the
>> shared zeropage. That will be posted soonish (within next 2 weeks).
>>
>
>Posted: https://lkml.kernel.org/r/[email protected]
>
>With that, we can use FAULT_FLAG_UNSHARE also to break COW on the shared
>zeropage.
Sounds a better way for breaking COW working with reliable R/O long-tern pinning.
>--
>Thanks,
>
>David / dhildenb