When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however
it may miss the failure when head page is unmapped but other subpage is
mapped. Then the second DEBUG_VM BUG() that check total mapcount would
catch it. This may incur some confusion. And this is not a fatal issue,
so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE().
[1] https://lore.kernel.org/linux-mm/[email protected]/
Reviewed-by: Zi Yan <[email protected]>
Signed-off-by: Yang Shi <[email protected]>
---
v4: Updated the subject and commit log per Hugh.
Reordered the patches per Hugh.
v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
since there is no fundamental change against v2.
v2: Removed dead code and updated the comment of try_to_unmap() per Zi
Yan.
mm/huge_memory.c | 24 +++++++-----------------
1 file changed, 7 insertions(+), 17 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 19195fca1aee..8827f82c5302 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2336,15 +2336,15 @@ static void unmap_page(struct page *page)
{
enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
- bool unmap_success;
VM_BUG_ON_PAGE(!PageHead(page), page);
if (PageAnon(page))
ttu_flags |= TTU_SPLIT_FREEZE;
- unmap_success = try_to_unmap(page, ttu_flags);
- VM_BUG_ON_PAGE(!unmap_success, page);
+ try_to_unmap(page, ttu_flags);
+
+ VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
}
static void remap_page(struct page *page, unsigned int nr)
@@ -2655,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
struct deferred_split *ds_queue = get_deferred_split_queue(head);
struct anon_vma *anon_vma = NULL;
struct address_space *mapping = NULL;
- int count, mapcount, extra_pins, ret;
+ int extra_pins, ret;
pgoff_t end;
VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
@@ -2714,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
}
unmap_page(head);
- VM_BUG_ON_PAGE(compound_mapcount(head), head);
/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
@@ -2732,9 +2731,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
/* Prevent deferred_split_scan() touching ->_refcount */
spin_lock(&ds_queue->split_queue_lock);
- count = page_count(head);
- mapcount = total_mapcount(head);
- if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
+ if (page_ref_freeze(head, 1 + extra_pins)) {
if (!list_empty(page_deferred_list(head))) {
ds_queue->split_queue_len--;
list_del(page_deferred_list(head));
@@ -2754,16 +2751,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
__split_huge_page(page, list, end);
ret = 0;
} else {
- if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
- pr_alert("total_mapcount: %u, page_count(): %u\n",
- mapcount, count);
- if (PageTail(page))
- dump_page(head, NULL);
- dump_page(page, "total_mapcount(head) > 0");
- BUG();
- }
spin_unlock(&ds_queue->split_queue_lock);
-fail: if (mapping)
+fail:
+ if (mapping)
xa_unlock(&mapping->i_pages);
local_irq_enable();
remap_page(head, thp_nr_pages(head));
--
2.26.2
Currently try_to_unmap() return bool value by checking page_mapcount(),
however this may return false positive since page_mapcount() doesn't
check all subpages of compound page. The total_mapcount() could be used
instead, but its cost is higher since it traverses all subpages.
Actually the most callers of try_to_unmap() don't care about the
return value at all. So just need check if page is still mapped by
page_mapped() when necessary. And page_mapped() does bail out early
when it finds mapped subpage.
Suggested-by: Hugh Dickins <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Signed-off-by: Yang Shi <[email protected]>
---
v4: Updated the comment of try_to_unmap() per Minchan.
Minor fix and patch reorder per Hugh.
Collected ack tag from Hugh.
include/linux/rmap.h | 2 +-
mm/memory-failure.c | 15 +++++++--------
mm/rmap.c | 7 +++----
mm/vmscan.c | 3 ++-
4 files changed, 13 insertions(+), 14 deletions(-)
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index def5c62c93b3..116cb193110a 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -194,7 +194,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
int page_referenced(struct page *, int is_locked,
struct mem_cgroup *memcg, unsigned long *vm_flags);
-bool try_to_unmap(struct page *, enum ttu_flags flags);
+void try_to_unmap(struct page *, enum ttu_flags flags);
/* Avoid racy checks */
#define PVMW_SYNC (1 << 0)
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 9dcc9bcea731..07e462588df9 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1063,7 +1063,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
enum ttu_flags ttu = TTU_IGNORE_MLOCK;
struct address_space *mapping;
LIST_HEAD(tokill);
- bool unmap_success = true;
+ bool unmap_success;
int kill = 1, forcekill;
struct page *hpage = *hpagep;
bool mlocked = PageMlocked(hpage);
@@ -1126,7 +1126,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
if (!PageHuge(hpage)) {
- unmap_success = try_to_unmap(hpage, ttu);
+ try_to_unmap(hpage, ttu);
} else {
if (!PageAnon(hpage)) {
/*
@@ -1138,17 +1138,16 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
*/
mapping = hugetlb_page_mapping_lock_write(hpage);
if (mapping) {
- unmap_success = try_to_unmap(hpage,
- ttu|TTU_RMAP_LOCKED);
+ try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
i_mmap_unlock_write(mapping);
- } else {
+ } else
pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
- unmap_success = false;
- }
} else {
- unmap_success = try_to_unmap(hpage, ttu);
+ try_to_unmap(hpage, ttu);
}
}
+
+ unmap_success = !page_mapped(hpage);
if (!unmap_success)
pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
pfn, page_mapcount(hpage));
diff --git a/mm/rmap.c b/mm/rmap.c
index a35cbbbded0d..d4e0d89726b8 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1749,9 +1749,10 @@ static int page_not_mapped(struct page *page)
* Tries to remove all the page table entries which are mapping this
* page, used in the pageout path. Caller must hold the page lock.
*
- * If unmap is successful, return true. Otherwise, false.
+ * It is the callers' responsibility to check if the page is still
+ * mapped when needed.
*/
-bool try_to_unmap(struct page *page, enum ttu_flags flags)
+void try_to_unmap(struct page *page, enum ttu_flags flags)
{
struct rmap_walk_control rwc = {
.rmap_one = try_to_unmap_one,
@@ -1776,8 +1777,6 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
rmap_walk_locked(page, &rwc);
else
rmap_walk(page, &rwc);
-
- return !page_mapcount(page) ? true : false;
}
/**
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f96d62159720..fa5052ace415 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1499,7 +1499,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
if (unlikely(PageTransHuge(page)))
flags |= TTU_SPLIT_HUGE_PMD;
- if (!try_to_unmap(page, flags)) {
+ try_to_unmap(page, flags);
+ if (page_mapped(page)) {
stat->nr_unmap_fail += nr_pages;
if (!was_swapbacked && PageSwapBacked(page))
stat->nr_lazyfree_fail += nr_pages;
--
2.26.2
On Wed, 26 May 2021, Yang Shi wrote:
> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however
> it may miss the failure when head page is unmapped but other subpage is
> mapped. Then the second DEBUG_VM BUG() that check total mapcount would
> catch it. This may incur some confusion. And this is not a fatal issue,
> so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE().
>
> [1] https://lore.kernel.org/linux-mm/[email protected]/
>
> Reviewed-by: Zi Yan <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Thanks: and 2/2 already has my Ack, correct.
> ---
> v4: Updated the subject and commit log per Hugh.
> Reordered the patches per Hugh.
> v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
> since there is no fundamental change against v2.
> v2: Removed dead code and updated the comment of try_to_unmap() per Zi
> Yan.
>
> mm/huge_memory.c | 24 +++++++-----------------
> 1 file changed, 7 insertions(+), 17 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 19195fca1aee..8827f82c5302 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2336,15 +2336,15 @@ static void unmap_page(struct page *page)
> {
> enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
> TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> - bool unmap_success;
>
> VM_BUG_ON_PAGE(!PageHead(page), page);
>
> if (PageAnon(page))
> ttu_flags |= TTU_SPLIT_FREEZE;
>
> - unmap_success = try_to_unmap(page, ttu_flags);
> - VM_BUG_ON_PAGE(!unmap_success, page);
> + try_to_unmap(page, ttu_flags);
> +
> + VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
> }
>
> static void remap_page(struct page *page, unsigned int nr)
> @@ -2655,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> struct deferred_split *ds_queue = get_deferred_split_queue(head);
> struct anon_vma *anon_vma = NULL;
> struct address_space *mapping = NULL;
> - int count, mapcount, extra_pins, ret;
> + int extra_pins, ret;
> pgoff_t end;
>
> VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
> @@ -2714,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> }
>
> unmap_page(head);
> - VM_BUG_ON_PAGE(compound_mapcount(head), head);
>
> /* block interrupt reentry in xa_lock and spinlock */
> local_irq_disable();
> @@ -2732,9 +2731,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>
> /* Prevent deferred_split_scan() touching ->_refcount */
> spin_lock(&ds_queue->split_queue_lock);
> - count = page_count(head);
> - mapcount = total_mapcount(head);
> - if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
> + if (page_ref_freeze(head, 1 + extra_pins)) {
> if (!list_empty(page_deferred_list(head))) {
> ds_queue->split_queue_len--;
> list_del(page_deferred_list(head));
> @@ -2754,16 +2751,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> __split_huge_page(page, list, end);
> ret = 0;
> } else {
> - if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
> - pr_alert("total_mapcount: %u, page_count(): %u\n",
> - mapcount, count);
> - if (PageTail(page))
> - dump_page(head, NULL);
> - dump_page(page, "total_mapcount(head) > 0");
> - BUG();
> - }
> spin_unlock(&ds_queue->split_queue_lock);
> -fail: if (mapping)
> +fail:
> + if (mapping)
> xa_unlock(&mapping->i_pages);
> local_irq_enable();
> remap_page(head, thp_nr_pages(head));
> --
> 2.26.2
>
>
On Wed, May 26, 2021 at 01:12:39PM -0700, Yang Shi wrote:
> Currently try_to_unmap() return bool value by checking page_mapcount(),
> however this may return false positive since page_mapcount() doesn't
> check all subpages of compound page. The total_mapcount() could be used
> instead, but its cost is higher since it traverses all subpages.
>
> Actually the most callers of try_to_unmap() don't care about the
> return value at all. So just need check if page is still mapped by
> page_mapped() when necessary. And page_mapped() does bail out early
> when it finds mapped subpage.
>
> Suggested-by: Hugh Dickins <[email protected]>
> Acked-by: Hugh Dickins <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Thanks for the update, Yang Shi.
On Wed, May 26, 2021 at 1:12 PM Yang Shi <[email protected]> wrote:
>
> Currently try_to_unmap() return bool value by checking page_mapcount(),
> however this may return false positive since page_mapcount() doesn't
> check all subpages of compound page. The total_mapcount() could be used
> instead, but its cost is higher since it traverses all subpages.
>
> Actually the most callers of try_to_unmap() don't care about the
> return value at all. So just need check if page is still mapped by
> page_mapped() when necessary. And page_mapped() does bail out early
> when it finds mapped subpage.
>
> Suggested-by: Hugh Dickins <[email protected]>
> Acked-by: Hugh Dickins <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
On Wed, May 26, 2021 at 01:12:38PM -0700, Yang Shi wrote:
> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however
> it may miss the failure when head page is unmapped but other subpage is
> mapped. Then the second DEBUG_VM BUG() that check total mapcount would
> catch it. This may incur some confusion. And this is not a fatal issue,
> so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE().
>
> [1] https://lore.kernel.org/linux-mm/[email protected]/
>
> Reviewed-by: Zi Yan <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
For both patches:
Acked-by: Kirill A. Shutemov <[email protected]>
--
Kirill A. Shutemov